BOSTON — More than two decades ago, Frank Enwonwu got caught smuggling five ounces of heroin into America from his homeland in Nigeria. He acknowledged his mistake and readily agreed to work as an informant, believing America had promised to keep him safe.

He went on to pursue his share of the American dream, driving a cab and training as a nurse’s aide — until a change in law in 1996 retroactively made him liable to be deported for his drug conviction, despite his work to help the government.

Now, he weeps in a room at a homeless shelter he shares with his 13-year-old son, fearful that any day he could be sent back to Nigeria to be tortured or killed as drug dealers with long memories seek retribution for his work as an informant.

“Trust me, no one there has forgotten what I did — even after 22 years. I’ll be killed there before I even have the ability to see daylight,” he said.

Mr. Enwonwu, 58, has spent about five of the last 11 years in detention while fighting his deportation order. His legal appeals all but exhausted, he now is asking to be spared on humanitarian grounds.

“I have a little boy who did not grow up with me because of all the time I have spent in detention. He needs me,” Mr. Enwonwu, who is separated from his wife and has custody of the teenager, said.

Mr. Enwonwu is under a final deportation order and could be taken into custody and deported without notice.

“This is a man who assisted the United States government as an informant, helping them prosecute drug-related crimes, and in so doing, he has put his life at complete risk. We believe that creates an obligation on the part of the United States to protect him,” an attorney at the American University Washington College of Law International Human Rights Law Clinic, Meetali Jain, said.

Mr. Enwonwu acknowledges he committed a crime when he brought drugs into America, but claims he was tricked by a Nigerian military officer who offered to buy him a plane ticket if he would show the man around Boston, where he had attended Tufts University in the 1970s.

The night of their flight, Mr. Enwonwu said, other military officers ordered him to carry two packets of heroin. He was arrested at Boston’s Logan International Airport after Customs officials found the drug.

Within hours of his arrest, Mr. Enwonwu said, federal drug agents asked him to participate in a sting to catch the dealers who were to come to Boston from New York to pick up the heroin. Mr. Enwonwu agreed, and two men were arrested. Their boss in Ohio was also prosecuted. All three were from Nigeria.

In the mid-1980s, Nigeria had become a major transit point for Asian heroin and South American cocaine being smuggled to Europe and North America. The transit networks expanded and became highly organized, prompting American pressure on Nigerian authorities to crack down on the trade, which Nigerian police say frequently involves gang killings.

Mr. Enwonwu worked for the Drug Enforcement Administration for 10 months, providing the names of suspected drug dealers in Nigeria who American officials believed were running drugs to America themselves or through couriers. He also supplied the names of Nigerians living in America who he had learned were involved with drugs. Mr. Enwonwu said the DEA promised him he would not be deported and would be protected from the drug dealers he had ratted on.

“They knew how dangerous the drug lords in Nigeria were and they told me I wasn’t going back to Nigeria,” Mr. Enwonwu said. “Based on that promise, I continued my cooperation with them.” The DEA acknowledges it paid him $1,600 for his work as an informant, but the DEA agent who Mr. Enwonwu claims made the promises, Herbert Lemon Jr., said he never told Mr. Enwonwu he would not be deported.

“Absolutely not. I [didn’t] have the authority to do it,” Mr. Lemon, who is now retired from the DEA, told the Associated Press. “That just didn’t happen.”

Mr. Lemon said he did tell federal prosecutors that Mr. Enwonwu had cooperated, which the agent believes spared Mr. Enwonwu from serving jail time. He got a suspended sentence and probation on the heroin charge.

“I think that’s the benefit he received for his helping the government,” Mr. Lemon said.

Mr. Lemon said he feels badly for Mr. Enwonwu’s wife and son who may be left behind in America, but said he does not fault the American government for now moving to deport Mr. Enwonwu. “He committed a criminal act, and as such, he has to face the consequences,” he said.