In a terrorism trial set to begin Monday, prosecutors will claim that the Pakistani immigrant accused of plotting to bomb a subway station was actively exchanging information on making explosives, according to a government letter submitted yesterday to U.S. District Court in Brooklyn.

The court filing raises the possibility that the alleged conspiracy to bomb the Herald Square subway station at 34th Street could have moved forward if police had not intervened. Since Shahawar Matin Siraj was arrested in August 2004, law enforcement officials have never claimed that Mr. Siraj, 23, had sought to make a bomb. But in the one-page letter, federal prosecutors allege that Mr. Siraj gave a computer disk containing instructions for making an “explosive device” to a police informant who posed as a co-conspirator.

The letter informs Mr. Siraj’s attorneys of the government’s intent to call an FBI explosives expert as a witness in the trial. That expert is expected to testify that the information contained in the disk could in fact be used to create explosives, according to the letter.

Mr. Siraj’s attorney, Martin Stolar, said he was skeptical that his client, who has appeared confused and slow-witted at some of his pretrial hearings, had the mental ability to produce a bomb. He said in a telephone interview that a court order prevented him from discussing evidence before it is presented in trial.

Mr. Siraj also goes by the name ‘Matin.’

“Their theory now is that Matin is the brains behind the operation and that Daoudi is along for the ride,” Mr. Stolar said, referring to the police informant, who is believed to have gone by the name Osama Daoudi. “Matin does not have the brains for the operation.”

Mr. Stolar has said Mr.Siraj will take the witness stand in his own defense. The lawyer has indicated at pretrial hearings that he will argue the police informant entrapped his client and goaded him into planning a terrorist act.

Although bomb-making guides are widely available on the Internet and at gun shows across the country, authorities had not previously suggested that Mr. Siraj, who lived in Queens, was developing the capability to make a bomb.

In the criminal complaint filed in the case, a NYPD detective who worked on the investigation states that Mr. Siraj and a co-defendant “never had possession or control” of a bomb. The complaint states that Mr. Siraj was told that the go-ahead to carry out the bombing would come through a fictitious Islamic “brotherhood” that never existed, and was a cover that the police informant created. Mr. Siraj had been led to believe that the informant would “provide any explosive devices” that would be used to bomb the station, the complaint indicates.

A jury has already been selected to hear the case and the prosecution is scheduled to begin its case Monday. A spokesman for the U.S. attorney’s office in Brooklyn did not return a phone call seeking comment about the letter.