A former Salvadoran army intelligence agent who is seeking asylum in the United States after alleging that U.S. military advisers knew of political killings carried out by his unit has been arrested in California on illegal immigration charges and could be deported to El Salvador.

Lawyers for Cesar Vielman Joya Martinez, who has been seeking political asylum since making his allegations last fall, said it was unprecedented for the government to bring criminal charges for illegal entry against a person who has filed for asylum. Asylum applications can only be made by those who have reached U.S. territory.

Joya Martinez was picked up by Immigration and Naturalization Service (INS) agents Tuesday in Long Beach.

Joya Martinez's allegations "raise very serious questions that need to be looked at," said Jim McGovern, spokesman for Rep. Joe Moakley (D-Mass.), who heads a task force on El Salvador. "The government should not be trying to shut him up and send him back."

Joya Martinez has said that as a member of the intelligence department of the Salvadoran army's First Brigade, he participated in seven or eight "death squad" operations in which he killed people.

He said two U.S. officers routinely worked in the brigade intelligence headquarters and knew details of the unit's day-to-day operations. The advisers, he said in an interview last October, were shown copies of arrest reports signed by superior officers but were not shown reports about executions of suspects.

David Carliner, who represents Joya Martinez in asylum proceedings, said his case is based in part on the possibility that his client would be killed if returned to El Salvador.

"If you have somebody fleeing for his life, you don't prosecute him for coming here," Carliner said, citing international law against forced repatriation. "I don't know of any such case; I think it's unprecedented."

Daniel S. Alcorn, Joya Martinez's lawyer in the criminal proceedings, said his research turned up no such reported cases.

INS spokesman Verne Jervis said the usual procedure when an illegal alien is caught is to handle any asylum request first, initiating deportation proceedings only if the asylum request is denied.

Joya Martinez's case is complicated by the fact that shortly after he surfaced in the United States and made his allegations last year, the Salvadoran government charged him with murder and said it would ask for his extradition for activities he says he carried out under orders as part of his military unit.

The asylum request is still pending. A spokesman for the U.S. Attorney's office declined to comment.

El Salvador, embroiled in a decade-long civil war that has taken more than 70,000 lives, has been receiving more than $500 million a year in U.S. military and nonmilitary aid. As an incentive to keep leftist rebels negotiating with the government, a Moakley amendment would cut the $85 million in military aid for fiscal 1991 in half, providing the full amount only if the rebels break off peace talks.

Joya Martinez, 28, was arrested on a bench warrant issued in Alexandria federal court last month after a grand jury returned an indictment charging him with reentering the country illegally. He was deported from the United States in 1983 as an illegal alien, fled El Salvador last July, and surfaced in Washington in October, where his cause has been taken up by human rights groups.

Meanwhile yesterday, two Congressmen have asked Attorney General Dick Thornburgh to investigate claims that the Bush administration is harassing critics of American policy in El Salvador.

On June 27, the Justice Department sent a letter to the National Agenda for Peace in El Salvador asking for information about its activities and telling the group that it may have to register under the Foreign Agents Registration Act for its "publicity" activities on behalf of the Farabundo Marti National Liberation Front (FMLN), the Salvadoran rebel opposition.

The letter "raises serious First Amendment concerns," Reps. Robert W. Kastenmeier (D-Wis.) and Don Edwards (D-Calif.) wrote Thornburgh on Wednesday, saying there was no evidence linking the group to the FMLN and asking for details on how the act is enforced and why the group was targeted.

The Justice Department letter cited an ad that ran in The Washington Post calling for a negotiated settlement to the war in El Salvador and suspension of U.S. aid for the government's failure to prosecute the murderers of six Jesuit priests. It was signed by dozens of religious leaders and activists, including Jesse L. Jackson and Coretta Scott King.

The National Agenda says it has no ties to the FMLN.

The American Civil Liberties Union, which released the Congressmen's letter yesterday, charges that the Justice Department letter harks back to the early days of the FBI's investigation of the Committee in Solidarity with the People of El Salvador, which ultimately led to the disciplining of six agents for overstepping their surveillance authority.

The ACLU asked that the Justice Department withdraw the request. Department spokesman Doug Tillet said, "We are just seeking information. If they are not in fact agents of a foreign principal, that would be the end of it." He declined to comment on the congressmen's letter.