The report, completed less than nine months before the Conservative government shut down the embassy and ordered Iranian diplomats out of Canada, said the biggest threat at the time was from a natural disaster.
"The high risk of catastrophic earthquake remains the most significant safety/security challenge for diplomatic missions in Tehran," says the report, which noted that high building density and poor local infrastructure in Iran's capital could lead a moderate earthquake to immediately kill 400,000 people.
The assessment was ordered after a 2011 attack on the British embassy compound, which raised concerns about the safety of Western officials and resulted in the U.K. closing its mission for nearly two years.
British officials complained the Iranians didn't do enough to stop protesters who stormed the compound, looting it and burning the British flag. The protest came the week after Britain, the U.S. and Canada hit Iran with new sanctions.
Foreign Affairs Minister John Baird cited concerns for the safety of Canadian diplomats in Tehran when he announced the closure in a news release more than two years ago, and later said he'd been considering it for months.
The Jan. 24, 2012, report was provided to CBC News under federal access-to-information laws. It was completed after meetings with top-level Iranian officials and consultation with experts and other embassies and written by an official posted in Tehran during a visit by an official from the security and intelligence bureau.
Large parts of it are blacked out for security reasons.
Long-held security concerns
At the time, Baird gave a number of reasons for the closing, including Canada's view that the Iranian regime was the most significant threat to global peace and security, and Iran's "blatant disregard" of the Vienna Convention that guarantees the protection of diplomatic personnel.
Canadian officials had long been concerned about how close to the road the embassy sat, among other security risks like sharing a wall with a neighbouring building.
The report notes political tensions between Iran and the international community had escalated over the previous six months, which caused some diplomatic observers to revise their risk assessments. It says there were growing but imprecise fears that the current confrontation between Iran and the international community could escalate into conflict.
"However, since Canada has already instituted tough sanctions without significant blowback, we do not believe that our own mission is a member of the 'front line' group," the report says.
At the same time, the report says, "in an attack similar to the U.K. embassy incident, no combination of security features or protocols would provide more than a buffer, allowing time for organization of a police intervention and destruction measures inside the chancery."
'No tangible threat'
The Canadians concluded the meeting with Iranian officials, including an assistant deputy minister for foreign affairs, was respectful.
The Iranians "clearly were hoping to ... reassure us on a range of mission security and operations issues," the report says.
"The relatively warm reception confirms the importance Iran places on maintaining some level of diplomatic ties with Canada and in particular the Iranian diplomatic presence in Canada."
Dan Livermore, who was director general of security and intelligence at the Department of Foreign Affairs from 2002 to 2007, says that's a shrewd assessment of the Iranians. He says it's crucial to have a presence there and that even the Israelis haven't stopped talking to them.
"There was no tangible threat," said Livermore in an interview with CBC News.
"The Iranians, for the terrible relations we had with them, they never closed their door to us," he said.
Livermore, who's now a senior fellow at the University of Ottawa's graduate school of international and public affairs, says the Conservative government doesn't believe in having a diplomatic presence in difficult countries.
"I don't know where they get their information, but on Iran it tends to be more rhetoric than material anchored in reality," he said.
Pro-democracy activists called for closure
A spokesman for Baird didn't answer specific questions about what changed between the January security assessment and the September closure of the embassy. He again cited the regime's "blatant disregard" for the protection of diplomatic personnel.
"Canada has held a clear position on Iran's sponsorship of terrorism, its nuclear ambitions, and its abysmal record on human rights. Diplomatic relations between Canada and Iran were suspended due to Canada’s position on the regime," Adam Hodge wrote in an email to CBC News.
"The government of Canada formally listed Iran as a state sponsor of terrorism under the State Immunity Act. It was under these circumstances that Canada could no longer maintain a safe or diplomatic presence in Iran."
At the time Canada shut down its mission in Iran, pro-democracy activists in Canada had been calling for the Iranian embassy in Ottawa to be closed. The calls were sparked by a July 2012 news report that said Iran's cultural counsellor in Ottawa, Hamid Mohammadi, suggested Iranian expatriates should be nurtured to be of service to Iran.
One of the most prominent voices calling for the closure was Nazanin Afshin-Jam, a human rights activist and the wife of then-defence minister Peter MacKay.