An unusual exchange of letters between an undercover informant and Senator Clinton has apparently angered authorities and potentially complicated the case of a man convicted of plotting to bomb a busy Manhattan subway station.

Osama Eldawoody — a paid NYPD informant who was instrumental in exposing the 2004 subway scheme — told the Associated Press that he wrote to Mrs. Clinton last month complaining that his law enforcement handlers misled him with false promises of a better life, and asking her to intervene on his behalf.

A written response from Mrs. Clinton, shared with the AP by Mr. Eldawoody, said she had “requested a review of this matter and a written response” from federal prosecutors in Brooklyn. The letter also suggests Mr. Eldawoody seek help from Senators Specter and Santorum of Pennsylvania, where he now lives.

Clinton spokesman, Philippe Reines, confirmed that her office had received the letter from Mr. Eldawoody, but said the senator was not personally involved in the written exchange and was unaware of the issue. He said caseworkers “simply forwarded it to the U.S. Attorney’s office, which we informed him of with a standard letter, as we routinely do with any constituent casework.”

Mr. Eldawoody, in a telephone interview last week, described being summoned to a meeting on August 31 with investigators and an assistant U.S. attorney in Brooklyn where he was reprimanded for contacting Mrs. Clinton and told not to do it again.

“Everybody was mad at me,” he said.

Mr. Eldawoody, 50, was a key government witness in the case against Shahawar Matin Siraj, who was convicted in May of plotting to blow up the Herald Square subway station. Police officials have touted the case as a success story for the NYPD’s heightened counterterrorism effort.

A U.S. attorney’s office spokesman, Robert Nardoza, declined to discuss Mr. Eldawoody. But Police Commissioner Raymond Kelly praised his work as an informant.

“He’s a hero who did a great service for the people of New York,” Mr. Kelly said in statement. “We will continue our dedicated efforts to assure the safety and well being of him and his family.”

Defense lawyers, arguing for a new trial in court papers filed last week, said prosecutors had turned over a letter “from a witness addressed to a high public official.” The letter, which was not made public, contained information that could undermine the government’s case, the defense papers said.

“It’s a piece of new evidence we didn’t know about,” an attorney for Siraj, Martin Stolar, said. He declined to elaborate.

The NYPD recruited Mr. Eldawoody, a naturalized U.S.citizen from Egypt, to monitor radical Muslims at mosques and elsewhere following the September 11, 2001, terrorist attacks. He testified at trial that he met Siraj at an Islamic bookstore near a Brooklyn mosque, and secretly recorded him over the next several months as his anti-American rants evolved into a plot to bomb the Herald Square subway station.

Since the trial ended, Mr. Eldawoody and his family have struggled to get by on a government stipend of $3,200 a month while living in seclusion in rural Pennsylvania, he said.

The informant said he had complained to Mrs. Clinton in his letter that, “The NYPD promised me the best quality of life and security … and all their promises were gone after the arrest” of Siraj. He closed the letter by asking, “Would you help me?” he said.

Mr. Eldawoody claimed that authorities also broke a promise to protect his identity. Once his name came out at the trial and was published in news accounts, he was told by friends that he was marked man in some Muslim communities.

“Do you know in Jersey City what they want to do with you?” he recalled the friends saying. “They want to butcher you.”