The federal government has sent letters to at least four people linked to extremist activity, including the man thought to be the mastermind of a plot to bomb downtown Toronto in an effort to terrorize Canadians and cripple the economy, telling them their citizenship is being revoked, CBC News has learned.

Zakaria Amara is among three members of the so-called Toronto 18 — the other two are Asad Ansari and Saad Khalid — who were informed by letter they were being stripped of citizenship under Bill C-24, dubbed the Strengthening Canadian Citizenship Act, passed last May.

Hiva Mohammad Alizadeh, convicted in an Ottawa terror plot and sentenced to 24 years in prison, received a similar letter.

Another man, Mohamed Hersi, received word from his parole officer that the government may be taking similar steps to revoke his citizenship, Hersi's lawyer, Paul Slansky told CBC News. Hersi has not, however, received any formal documents.

Hersi was convicted of trying to join the Somali militant group al-Shabaab and received a 10-year sentence in July 2014.

CBC News is working to confirm reports that others linked to extremist activity have received similar letters.

This man hated Canada so much, he planned on murdering hundreds of Canadians. He forfeited his own citizenship. <a href="https://t.co/gxqDtOoKUg">https://t.co/gxqDtOoKUg</a> —@jkenney

Recipients of a notice of citizenship revocation were given 60 days to respond to it.

Amara was sentenced in 2010 to life in prison with no chance of parole until 2016 after admitting his role in the plan to attack sites in Toronto, aimed in part at forcing Canadian soldiers to leave Afghanistan. His citizenship has already been revoked, after receiving the letter in June.

Ansari was released with time served in 2010 for his role in the Toronto 18 plot. He was convicted of knowingly contributing to, directly or indirectly, a terrorist group for the purpose of enhancing the ability of the group to carry out an act of terror.

Khalid is still serving time for intending to cause an explosion that would likely cause serious bodily harm, death or damage to property.

At least two of the men applied for delays in responding to the notices that their citizenship is being revoked, as they plan to challenge the constitutionality of the move.

Ansari's lawyer, John Norris, said his client was granted an extension. Alizadeh's lawyers, Howard Krongold and Leo Russomamo, said their client received the letter in June, but notice to respond has been delayed until December.

Following a National Post report published Saturday saying Amara's citizenship will be pulled, Defence Minister Jason Kenney sent a tweet describing him as a man who hated Canada so much that he "forfeited his own citizenship" by plotting to murder hundreds of Canadians.

According to the Post, he was notified of the decision on Friday in a letter sent to the Quebec penitentiary where is he is serving a life sentence. He still holds citizenship in Jordan, where he was born.

Bill C-24 allows the government to revoke someone's citizenship for serious acts against Canada, providing that person is a dual national and is convicted of offences related to spying, treason or terrorism.

Police thwarted the Toronto 18 plot when they arrested Amara and 17 other people in the summer of 2006.

'Violent disloyalty'

"If somebody is found guilty of violent disloyalty to Canada, in this instance planning to murder hundreds or potentially thousands of Canadians for ideological reasons, that they are in so doing forfeiting their Canadian citizenship," Kenney said in an interview with The Canadian Press.

"They are renouncing through their actions, through their own violent volition, they are renouncing their Canadian citizenship."

Police thwarted the plot when they arrested Amara and 17 other people in the summer of 2006.

In an agreed statement of facts, Crown lawyer Ione Jaffe told court that Amara planned to rent U-Haul trucks, pack them with explosives and detonate them via remote control in the Toronto area.

The Mississauga, Ont., man said the attack, which he planned over three consecutive days to maximize the panic, also involved bombing RCMP headquarters, nuclear-power plants and attacking Parliament.

The group also considered attacking the Sears Tower in Chicago or UN headquarters in New York three months after the proposed "Battle of Toronto," court heard.

"This man hated Canada so much, he planned on murdering hundreds of Canadians," Kenney tweeted on Saturday. "He forfeited his own citizenship."

Amara, who was married with one child at the time, planned to flee to Pakistan around the time of the blast and then move on to Afghanistan.

Admitted to organizing training camp

He also admitted to a leadership role in organizing a winter camp north of Toronto in December 2005 in which "recruits" were given basic combat training along with indoctrination in the extremist jihadi cause.

On several calls with his confidants, Amara acknowledged that he risked a lengthy jail term but said he "won't feel sorry" if arrested as long as he had "tried his best."

Through an undercover police agent, Amara attempted to buy large quantities of ammonium nitrate — commonly used in fertilizer — and other chemicals to build the bombs, court heard.

Police pounced on June 2, 2006, when an undercover agent delivered 120, 25-kilogram bags labelled ammonium nitrate but containing a harmless substance to a warehouse in Newmarket, Ont., rented by the plotters.

RCMP explosives experts determined a one-tonne truck bomb would have caused "catastrophic" damage to a high-rise building 30 metres away and death and injury to anyone nearby.

Amara pleaded guilty to two counts — knowingly participating in a terrorist group, and intending to cause an explosion that could kill people or damage property for the benefit of a terrorist group.