Opposition parties are calling for an inquiry into accusations that Canada's spy agency blocked a police investigation into the bombing of an Air India plane.

The Canadian Security Intelligence Service tried to conceal that one of its agents was close to the plot, according to secret RCMP files just released.

CSIS ordered the destruction of hundreds of wiretaps to cover up any evidence that it knew about the planned bombing, the Mounties believe.

Surjan Singh Gill

A CSIS mole penetrated a circle of Sikh extremists accused of planning the attack, according to the files. It's not clear exactly what he told the spy agency before the bombing.

Investigators say the informant, identified as Surjan Singh Gill, was apparently aware that something was about to happen. He was ordered to pull out three days before Air India Flight 182 blew up in June 1985, killing 329 people.

Opposition MPs are demanding an inquiry into how much CSIS knew about the impending tragedy, and why the organization did not warn police. Two B.C. men are now on trial for the bombing – the worst case of mass murder in Canadian history.

"There are very profound questions about just how deeply involved the CSIS source was," says NDP justice critic Svend Robinson. "What did he know?"

"Obviously, they were well aware that (the bombing) was going to happen," says Alliance MP Randy White. "The question is, then, 'Why wouldn't they have passed that information over to the RCMP, possibly preventing this whole bombing disaster in the first place?'"

By erasing wiretaps, it appears that CSIS was "prepared to effectively jeopardize a successful prosecution" just so that its informant was not implicated, Robinson says.

The RCMP files also raise concerns about the destruction of evidence. The loss could sabotage the entire prosecution and result in acquittals, according to one police report. It predicts "intense criticism of CSIS" when all the facts come out a trial.