A former Canadian resident arrested in the U.S. allegedly "discussed" plans to kill tens of thousands of people through air and water contamination with a Montreal man already accused in a cross-border terror plot.

Tunisian national Ahmed Abassi has been charged with fraudulently applying for a visa in order to remain in the U.S. to facilitate an "act of international terrorism," U.S. authorities said Thursday.

Prosecutors allege Abassi had "radicalized" Chiheb Esseghaier, one of two men charged with plotting to attack a Via Rail train travelling from the U.S. to Canada.

Prosecutors said Abassi told an undercover FBI agent that Esseghaier's plans were good but the time was not right.

"The defendant noted that he had suggested an alternative plot — contaminating the air or water with bacteria in order to kill up to 100,000 people — but that Esseghaier was dismissive of that plan," prosecutors said.

Officials said Abassi travelled from Canada to the U.S. in mid-March, where he was arrested at JFK airport on April 22 — the same day Canadian authorities arrested Esseghaier and Raed Jaser, 35 of Toronto.

Abassi appeared before a New York judge May 2 and pleaded not guilty.

"Ahmed Abassi had an evil purpose for seeking to remain in the United States – to commit acts of terror and develop a network of terrorists here, and to use this country as a base to support the efforts of terrorists internationally," said Preet Bharara, U.S. Attorney for the Southern District of New York.

"Thanks to the extraordinary vigilance of our prosecutors and law enforcement partners, Abassi has been thwarted and is being prosecuted for his alleged crimes."

The FBI alleged Abassi was unaware that one of his associates was an undercover FBI agent who was privy to the details of the terror plot. Authorities said Abassi was under constant surveillance while in the U.S.

Abassi met with Esseghaier in New York City, said a release jointly issued by the U.S. Attorney's Office, the FBI and the New York Police Department.

"Esseghaier, who was recently arrested in Canada and is currently incarcerated there on terrorism charges, was previously radicalized by Abassi," the release said.

"Abassi discussed his desire to engage in terrorist acts against targets in the United States and other countries, and his intention to provide support and "proposed terrorist plots."

"The defendant, among other things...explained that he and Esseghaier discussed plots to poison a water system and to derail a passenger train," the document funding to organizations engaged in terrorist activity."

According to U.S. court documents, Abassi allegedly discussed Esseghaier's "proposed terrorist plots."

"The defendant, among other things...explained that he and Esseghaier discussed plots to poison a water system and to derail a passenger train," the document said.

Esseghaier allegedly later "received overseas training and plotted to engage in terrorist activity in the West."

Esseghaier, also a Tunisian national, was a PhD student doing doctoral research on nanosensors in Montreal.

He is facing five charges, including instructing someone to carry out an activity for the benefit of a terrorist group. He suggested in a Toronto court last month that he doesn't recognize the authority of the Criminal Code because it is "not a holy book.''

The FBI alleged that Abassi told the undercover officer he knew of a number of individuals who, like him, would be willing to engage in terrorist activity.

He was recorded suggesting they send money and weapons to Muslims fighting in Syria, the court documents said.

Additionally, documents said Abassi allegedly expressed his desire to support terror groups, including the al Nusrah Front, which is recognized by the U.S. government as an alias for al-Qaida in Iraq.

"Abassi made clear that he wanted to obtain immigration documents and to remain in the United States so that he could engage in 'projects' relating to future terrorist activities, including recruitment," authorities said.

Details on Abassi's case came after an indictment and other documents were unsealed in Manhattan federal court Thursday.

The indictment charges Abassi with two counts of knowingly making false statements in an application to the immigration authorities for a green card and work visa in order to facilitate an act of international terrorism.

Each count carries a maximum term of 25 years in prison.

Meanwhile, Esseghaier and Jaser could be sentenced to life in prison if convicted of the charges against them.

Jaser's lawyer has said his client "is in a state of shock and disbelief," and denies all the allegations against him.

Both Jaser and Esseghaier are set to next appear in court via video on May 23. Abassi has another court appearance on June 11.

The RCMP said Thursday that it had worked "very closely" with the FBI.

"The FBI's parallel investigation has led ultimately to the laying of charges against the individual in the US," said Sgt. Greg Cox. "Through our efforts, we assured that at no time did this individual pose an imminent threat to public safety in Canada."

Cox added that authorities wouldn't be providing further details on any of the three men arrested in connect with the plot as their cases were before the courts.

Canadian authorities have said that those accused in the train terror plot had the capacity to carry out an attack, but there was no imminent threat' to the public. They said authorities uncovered the plot thanks to tips from the Muslim community.

The RCMP has said the plot was guided and supported by al-Qaida elements in Iran but they don't believe it was state-sponsored.

Meanwhile, Via Rail said Thursday it's contemplating whether to ask all of its travellers for identification as it considers ways to improve security.

Transport Canada said it expects railway organizations to make appropriate threat assessments and develop measures to prevent problems.

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  • Chiheb Esseghaier

    Chiheb Esseghaier, one of two suspects accused of plotting with al-Qaeda in Iran to derail a train in Canada, arrives at Buttonville Airport just north of Toronto, on Tuesday, April 23, 2013. Canadian investigators say Raed Jaser, 35, and his suspected accomplice Esseghaier, 30, received "directions and guidance" from members of al-Qaeda in Iran. In a brief court appearance in Montreal Tuesday, Esseghaier declined to be represented by a court-appointed lawyer. He made a brief statement in French in which he called the allegations against him unfair.

  • John Norris, the attorney for accused Raed Jaser scrums with the media at Toronto's Old City Hall court house.

  • Chiheb Esseghaier

    In this courtroom sketch, Chiheb Esseghaier appears in court in Montreal on Tuesday, April 23, 2013. Esseghaier, 30, and Raed Jaser, 35, were arrested and charged Monday in what the RCMP said was the first known al-Qaeda terror plot in Canada.

  • Family members of Raed Jaser leave court in Toronto on Tuesday, April 23, 2013. Jaser, a man accused with another man of plotting to derail a train in Canada with support from al-Qaida elements in Iran made a brief court appearance and was told to appear in court again next month.

  • Chiheb Esseghaier

    Chiheb Esseghaier, one of two suspects accused of plotting with al-Qaida in Iran to derail a train in Canada, arrives at Buttonville Airport just north of Toronto, on Tuesday, April 23, 2013. Canadian investigators say Raed Jaser, 35, and his suspected accomplice Esseghaier, 30, received "directions and guidance" from members of al-Qaida in Iran. In a brief court appearance in Montreal Tuesday, Esseghaier declined to be represented by a court-appointed lawyer. He made a brief statement in French in which he called the allegations against him unfair.

  • Chiheb Esseghaier

    Chiheb Esseghaier, one of two men accused of plotting a terror attack on rail target, is led off a plane by an Royal Canadian Mounted Police (RCMP) officer at Buttonville Airport just north of Toronto on Tuesday April 23, 2013. Canadian investigators say Raed Jaser, 35, and his suspected accomplice Esseghaier, 30, received "directions and guidance" from members of al-Qaeda in Iran. In a brief court appearance in Montreal Tuesday, Esseghaier declined to be represented by a court-appointed lawyer. He made a brief statement in French in which he called the allegations against him unfair.

  • Chiheb Esseghaier is taken off an airplane at Buttonville Airport in Markham, Ont. on Monday April 22, 2013.

  • John Norris, the lawyer for Raed Jaser, one of the two men accused of plotting a terror attack on a Canadian rail target, leaves court in Toronto on Tuesday, April 23, 2013. Jaser, 35, was charged in Toronto Tuesday in an alleged al-Qaeda supported terror plot to attack a Via passenger train. His suspected accomplice Chiheb Esseghaier, 30, was charged in Montreal.

  • Mohammed Jaser, father of Raed Jaser, leaves court in Toronto on Tuesday, April 23, 2013. Raed Jaser is accused with another man of plotting to derail a train in Canada with support from al-Qaeda elements in Iran. Raed Jaser had a brief court appearance and was told to appear in court again next month.

  • Security officials check a man at a courthouse in Montreal on Tuesday, April 23, 2013. Reed Jaser, one of two men accused of plotting a terrorist attack against a Canadian passenger train with support from al-Qaida elements in Iran, made a brief court appearance Tuesday but did not enter a plea. Canadian investigators say Jaser, 35, and his suspected accomplice Chiheb Esseghaier, 30, received “directions and guidance” from members of al-Qaida. The case prompted an immediate response from Iran, which denied any involvement and said groups such as al-Qaida do not share Iran’s ideology.

  • Security officials check a man at a courthouse in Montreal on Tuesday, April 23, 2013. Reed Jaser, one of two men accused of plotting a terrorist attack against a Canadian passenger train with support from al-Qaeda elements in Iran, made a brief court appearance Tuesday but did not enter a plea. Canadian investigators say Jaser, 35, and his suspected accomplice Chiheb Esseghaier, 30, received “directions and guidance” from members of al-Qaeda . The case prompted an immediate response from Iran, which denied any involvement and said groups such as al-Qaeda do not share Iran’'s ideology.

  • An RCMP officer shakes hands to what appears to be pilots after a transfer of a terror suspect at Buttonville Airport, April 22, 2013.

  • A Royal Canadian Mounted Police officer pats a colleague on the back before a press conference in Toronto as the RCMP announce the arrest of two men accused of plotting a terror attack on rail target on Monday April 22, 2013.

  • RCMP officers stand outside the Toronto home of one of the two men accused of plotting a terror attack on a rail target, on Monday April 22, 2013.

  • Officers from various law enforcement agencies including the Royal Canadian Mounted Police, Peel Regional Police, and Surete du Quebec gather at a press conference in Toronto, Monday, April 22, 2013 as the RCMP announce the arrest of two men accused of plotting a terror attack on rail target on Monday April 22, 2013.

  • Officers from various law enforcement agencies including the Royal Canadian Mounted Police, Peel Regional Police, and Surete du Quebec gather at a news conference in Toronto on Monday, April 22, 2013, as the Royal Canadian Mounted Police announce the arrest of two men accused of plotting a terror attack on a rail target.

  • Royal Canadian Mounted Police Sgt. Laporte walks with Mohammad Shaied Sheikh of the Masjid el Noor Mosque before attending a news conference in Toronto, Monday, April 22, 2013, as the Royal Canadian Mounted Police announce the arrest of two men accused of plotting a terror attack on rail target.

  • Representatives of Toronto's Islamic community attend a news conference in Toronto as the Royal Canadian Mounted Police announce the arrest of two men accused of plotting a terror attack on rail target, in Toronto, Monday April 22, 2013.

  • MORE: Twitter Reacts To Alleged Canadian Terror Plot