The government's star witness in the trial of three terrorism suspects was grilled by a defence lawyer in a Brampton, Ont., courtroom on Tuesday.

Mubin Shaikh's testimony is a key part of the government's case against the men, who are accused of plotting terror attacks against the Parliament buildings in Ottawa and other targets in Ontario.

Fahim Ahmad, 25, Asad Ansari, 25, and Steven Chand, 29, are facing charges of participating in a terrorist group that came to be known as the Toronto 18. Each has pleaded not guilty to all charges.

Ahmad is also charged with instructing people to carry out activities for a terrorist group and a weapons offence. Chand also faces a charge of counselling to commit fraud over $5,000 for the benefit of a terrorist group.

On Tuesday, defence lawyer Dennis Edney concluded his cross-examination of Shaikh, who worked as an undercover agent to infiltrate the alleged terrorist group.

Shaikh had testified that he trained the others to use a 9-mm handgun at the winter camp in Washago, Ont., attended by co-accused Ahmad, Chand and Ansari.

Shaikh also testified he had purchased camping supplies and drove Ahmad up to the camp but didn't inform his government handlers at the time that there was a loaded gun at the site.

Shaikh's credibility attacked

Edney suggested his client Ahmad was a fantasist and that Shaikh — who was 30 years old at the time — was egging on the younger and gullible men.

An attorney from Alberta, Edney is no stranger to cases involving terror suspects. He serves as the Canadian lawyer for Omar Khadr, who the U.S. is trying at Guantanamo Bay, Cuba, for allegedly killing an American soldier in Afghanistan.

He also represents Abdullah Khadr — once accused of being an al-Qaeda commander.

Under Edney's cross-examination, which began last week, Shaikh insisted his task was to get close to the group and its alleged ringleader, Fahim Ahmad.

He said to succeed he had to maintain his cover and that meant passing certain tests and improvising. Shaikh said Ahmad — who was 21 at the time — had "already positioned himself as a leader" in the group.

In another development, the trial judge, who cannot be named for security reasons, addressed the jury on the issue of entrapment. While he didn't place limitations on questions by the defence, he did remind the jury that its role is to assess evidence and decide on the credibility and the reliability of a witness such as Shaikh.

Any decision regarding entrapment — the idea that the government created a crime that would not otherwise have been committed — is something that will be determined at the end of the trial, the judge said.

With files from CBC's Stephanie Matteis