New York ? When Mohamed Alanssi set himself on fire in a suicide attempt in front of the White House this week, he blew his cover as an FBI informant and opened a window on how the government is fighting the war on terror.

But the consequences of his actions do not stop there: Alanssi also may have damaged the investigations he was working on, including a case against a firebrand Yemeni sheik caught on tape bragging about giving millions of dollars to Osama bin Laden. Atty. Gen. John Ashcroft has billed the case as a key effort in a campaign to cut off funding for terrorists.

Defense lawyers for the sheik and other suspects said the episode undermined Alanssi’s credibility and called into question his mental stability. They also have seized on reports that claimed the FBI promised to make him rich in exchange for his help; defense lawyers say that gave Alanssi a motive to ensnare the defendants.

“He is the man who is the contact between the government and my client,” said Howard Jacobs, the attorney for Sheik Mohammed Ali Hasan al-Moayad. “His credibility is at great issue in the case.”

Prosecutors have declined to discuss Alanssi but insist they have enough evidence to convict the sheik, a leading member of an Islamic-oriented political party in Yemen, and his assistant when they go on trial in January, even if the informant does not testify.

Details about the 52-year-old Alanssi — and the reasons for his attempted suicide — were sketchy. In a note to The Washington Post, a distraught Alanssi had warned of a suicide attempt. Alanssi remains hospitalized with burns over 30 percent of his body.

Alanssi was a key ally of the government in the case against the sheik, helping lure the cleric to a bugged hotel room in Germany where he was arrested. The informant’s next assignment was to testify against the sheik at an upcoming trial in New York.

The case began in 2002 with a roundup of more than a dozen small businessmen in an Arab enclave in Brooklyn. The men were charged with illegally transmitting money to Yemen. FBI agents believed the underground financial network could lead them to high-placed terror suspects. And the FBI persuaded Alanssi to help out.

Alanssi was particularly suited for the job: He once was a neighbor of the sheik in Yemen, and worshipped at his mosque, according to the FBI.

Mohamed Alanssi screams after being burned Monday in front of the White House in Washington, D.C. Alanssi, who set himself on fire, had burns to his head, back, arms and face but was conscious when medics took him to Washington Hospital Center, emergency spokesman Alan Etter said. He remained hospitalized Friday.

The FBI dispatched Alanssi to his native land, where he re-established contact with al-Moayad. The informant soon reported that al-Moayad had confided in him about his ties to al-Qaida and the Palestinian militant group Hamas.

Alanssi’s handlers devised a sting: He would tell the sheik he knew a wealthy American Muslim who wanted to donate to Islamic fundamentalist causes. A meeting was arranged in January 2003 at a hotel near the Frankfurt airport in Germany. In attendance: the American, Alanssi, al-Moayad and the sheik’s assistant.

Al-Moayad allegedly told the American, also an informant, that he had personally delivered $20 million to bin Laden before the Sept. 11 terrorist attacks.

Al-Moayad also allegedly named four men in New York he said had secretly transferred funds to him in Yemen. Alanssi returned to Brooklyn to help the FBI investigate the men, including a Yemeni-born ice cream shop owner now facing charges of illegally transferring $20 million overseas.

Alanssi eventually resettled in Virginia to wait for the trial.

But Alanssi soon grew restless, complaining that he was being denied his request to visit his ailing wife and family in Yemen.

“Why don’t you care about my life and my family’s life?” he wrote to an FBI agent. “Once I testify my family will be killed in Yemen, me too I will be dead man.”

Finally, he warned that he was “going to burn my body” at an unexpected place.