The RCMP, they were informed, would like them to clear their schedules for an afternoon meeting.
It wasn't a cold call; many of the groups had long-standing ties with Canadian security agencies as part of ongoing outreach on the part of police with Muslim organizations.
Still, those who received the e-mail said it was a bit unnerving, more so when a second message came in an hour later giving the time and location of the sit-down meeting: 2:30 p.m. at RCMP offices in Toronto.
A public press conference had been scheduled for an hour later.
"I was pretty certain it was something big," said Muhammad Robert Heft, who runs an organization that provides outreach to Islamic converts.
It wasn't the first time that Muslim leaders had been so summoned by the police; ahead of the arrests in the Toronto 18 case in 2006, a similar gathering had been convened.
As Heft and others crowded into the boardroom Monday, the buzz was around wondering exactly what the police were about to announce.
"People knew that this meeting could only mean there were arrests," said Hussein Hamdani, a Hamilton, Ont., lawyer and longtime advocate in the Muslim community.
"But there was fear and stress about what the arrests would be."
Police led off the meeting thanking the Muslim community for their work before sombrely informing them that two men had been arrested for plotting to attack a Via Rail passenger train.
They divulged their names and ages, as well as the fact that one is Tunisian and the other is from the United Arab Emirates.
Police said they had Chiheb Esseghaier and Raed Jaser under surveillance since August. They were put on investigators' radar thanks to tips from the Muslim community.
Neither of the men are Canadian and the gathering wasn't told how and when they arrived in the country.
"We understood and we were happy that the RCMP did their due diligence and was able to stop anything from happening, first and foremost," Heft said.
"And the mood there was what will be the backlash, how are we going to handle the media and the press reports from looking like they are stereotypically demonizing Muslims, when it was a tip from our leadership that prompted the arrest."
Those in the room represented all facets of Canada's Islamic community, from mosques to secular organizations.
All have felt keenly in the past the blowback that's come after Muslims are implicated in terrorist activities.
The leaders received a 30 minute briefing before the floor was opened for questions.
Much of the time was spent thanking the RCMP and on further discussion on outreach, but there was some skepticism.
Some wondered about the timing of the arrests, coming on the same day that counter-terrorism legislation was up for debate in the House of Commons.
The briefing was held almost at the exact same time that twin explosions shook the Boston marathon one week ago.
The RCMP insisted there was no connection, Hamdani said, and it was just a co-incidence.
Meetings such as Monday's go further than people realize in the RCMP's counter-terrorism efforts, Heft suggested.
It helps send a signal that police aren't targeting the community, he said.
"It says enough for guys like me to go back to people and say no, no that's not how it is. You might think it's this way, you might try to come up with a conspiracy but I know some real stand-up guys inside the RCMP who took the time to take our community aside and say 'hey look, you guys are part of the greater community, there might be things said about you guys but know that's not what we believe,'" Heft said.
Also on HuffPost