Khurram Sher, Hiva Alizadeh and Misbahuddin Ahmed have been charged in connection with an alleged domestic terrorism plot. Police have also arrested a fourth suspect. (Sarah Wallace/CBC)

Police say they have arrested a fourth person in their investigation into an alleged domestic terrorist plot, but no charges have been laid.

Earlier Friday in Ottawa, the RCMP's Integrated National Security Enforcement Team "executed one search warrant and took one person into custody as part of our standard operating procedures in the course of the search," said RCMP spokesman Marc Ménard. "No charges against this individual have been laid."

Menard said police would not release any person's name until charges have been laid.

But a police source told CBC News the individual arrested, who was identified as a man, is not likely to be charged. The source told CBC News there does not appear to be enough evidence for a charge and police are unlikely to hold the individual under anti-terrorism legislation.

Authorities have arrested and charged three Ontario men in what the RCMP is calling a conspiracy to commit "a violent terrorism attack."

Earlier Friday, Khurram Sher, 28, of London, Ont., was remanded in custody until Sept. 1 after a brief court appearance. He was charged Thursday with conspiracy to knowingly facilitate a terrorist activity.

Two Ottawa men, Misbahuddin Ahmed, 26, and Hiva Alizadeh, 30, were arrested on Wednesday. They appeared in an Ottawa courtroom on Thursday facing the same charge.

Alizadeh is also charged with being in possession of an explosive substance with intent to harm and providing property or financial services for the benefit of a terrorist group.

The men discussed specific targets in Canada, according to security sources CBC spoke with, including specific government buildings and transit systems, but didn't mention any of those targets by name.

Former senior CSIS officer Michel Juneau-Katsuya told CBC News his sources had said Parliament Hill was among the targets discussed. Juneau-Katsuya also suggested Montreal's transit system was a possible target because two of the men had roots in the city, and had lived and studied there.

None of the targets was in the United States, sources said.

During a Thursday news conference, RCMP Chief Supt. Serge Therriault said part of the decision to make the arrests now was to prevent one of the suspects from providing financial support to terrorist counterparts abroad.

The RCMP investigation, dubbed Project Samossa, found evidence that one member of the group had been trained to construct electronic and explosive devices.

Alleged plan to build makeshift bombs

During their investigation, Therriault said, police seized more than 50 electronic circuit boards they say were designed specifically to remotely detonate improvised explosive devices, or IEDs.

CBC News has learned that the RCMP's Integrated National Security Enforcement Team knew about the circuit boards some time ago — for perhaps months or at least many weeks.

The team obtained a warrant to enter Alizadeh's apartment and surreptitiously removed the boards, replacing them with look-alikes that were duds. Therefore any attack would likely have failed.

Therriault said they also seized a vast quantity of terrorist literature, videos and manuals.

"This group posed a real and serious threat to the citizens of the National Capital Region and Canada's national security," he said.

The RCMP allege that the year-long investigation turned up evidence that the three men conspired with three other men, whom they named as James Lara, Rizgar Alizadeh and Zakaria Mamosta (who Reuters reported are not in Canada), and other unnamed individuals in Canada, Iran, Afghanistan, Pakistan and Dubai to commit terrorism-related offences.

A security source told CBC News that the men mostly communicated over the internet. But according to a document seen by CBC News, police made surveillance recordings at the Ottawa townhouse rented by Misbahuddin Ahmed.

People were allegedly overheard praying together and talking about Canada's anti-terrorism law, the structure of terror cells and the so-called Toronto 18.

Investigators said they have reason to believe Hiva Alizadeh is a member of, and in contact with, a terrorist group with links to the conflict in Afghanistan but declined to name the group.

Suspects are educated professionals

All three suspects arrested are educated men pursuing professional careers. Police said Sher is a McGill medical graduate who travelled to Pakistan in 2006 to help with earthquake relief and also auditioned for the Canadian Idol singing competition in the past.

Ahmed worked as an X-ray technician at an Ottawa hospital and had a wife and child, while Alizadeh had studied to be an electrical engineer. All three men are Canadian citizens.

Their profiles are likely to raise concerns about homegrown radicalism, said security expert Eric Margolis, who said the roots of the radicalism are likely triggered by anger over the involvement of Western governments in countries such as Afghanistan.