After New York Police Department investigators came to Osama Eldawoody’s door, the immigrant from Egypt decided to become a police informant. Both as a Muslim and as a patriotic citizen, he said he was eager to help the police after September 11.

“I asked, ‘Why all this discrimination?'” Mr. Eldawoody, 50, said from the witness stand yesterday, recalling his conversation with three police officers who appeared at his door in October 2002. Mr. Eldawoody remembers the response one of the officers gave him:

“He answered me, ‘These days were really strange days.’ When he said that, I felt something had to be done. … Right away, I said I was ready to help.”

He was soon on the NYPD payroll, visiting mosques and telling his police handler what the imams were saying to worshippers. He jotted down the license plate numbers of the cars of those who came to pray. Cursing America was commonplace at mosques, but calls to violence were not, he reported.

Then Mr. Eldawoody, who has a degree in nuclear engineering, met a clerk at an Islamic Bookstore on 5th Avenue in Bay Ridge. After a few conversations, the young clerk asked if Mr. Eldawoody knew how to make a ‘dirty bomb,’ he said.

The trial of that man, Shahawar Matin Siraj, 23, began yesterday in U.S. District Court in Brooklyn. Mr. Siraj, a Pakistani immigrant who was arrested in August, 2004, is charged with plotting to bomb the Herald Square station at 34 Street. Mr. Siraj had considered other targets too, prosecutors allege.

“In the summer 2004 he could barely pass a bridge or subway station without planning to place to bomb there,” Assistant U.S. Attorney Todd Harrison, referring to Mr. Siraj, said.

Mr. Siraj’s attorney, Martin Stolar presented a different chronology, claiming that Mr. Eldawoody served as the catalyst for the bomb plot. The lawyer said Mr. Siraj began plotting violence only after Mr. Eldawoody goaded the defendant into hating America.

Chance, mixed with a good deal of legwork, may have brought Mr. Eldawoody into contact with Mr. Siraj. The bookstore where Mr. Siraj worked was adjacent to the Islamic Society of Bay Ridge, one of the mosques that Mr. Eldawoody attended as part of his work for the police.

At the bookstore, Mr. Eldawoody said Mr. Siraj began showing him various documents on a laptop computer: pictures of torture at the Abu Ghraib prison and instructions for making bombs. There also was a sketch of Staten Island that notes four bridges, three police precincts, and a jail.

“He used to ask me, ‘Do you know how to design a nuclear bomb?,” Mr. Eldawoody said of Mr. Siraj. “Sometime he used the word ‘dirty bomb.’ My answer is ‘Yes.'”

In his opening arguments, Mr. Stolar said the NYPD paid the informant, who no longer lives in the state, about $100,000.

“Eldawoody continued to talk with him about engaging in violence because he needs someone to make money off of,” Mr. Stolar said. “So he goes to a young, pliable man, not the brightest light bulb in the chandelier. … He says it’s your duty as a Muslim to do something.”

In his opening, Mr. Stolar also said that Mr. Siraj never possessed explosives, adding that the plot called for those to be supplied by Mr. Eldawoody.

“We’re dealing with a plot that never would have succeeded,” Mr. Stolar said. “Nobody was ever going to get hurt.”