An American adviser to the Salvadoran National Police was shot and killed today in an apparent shootout with at least one leftist guerrilla, authorities here said.

The U.S. Embassy here confirmed the death of Thomas Bracken, about 40 years old, but said he was not in El Salvador in any official U.S. capacity and was in fact unknown to U.S. officials until he visited the embassy on a consular matter last week.

A report issued this evening by the Armed Forces Information Commission said Bracken was helping the police investigate the kidnapping of Rene Machon, the president of the National Coffee Institute who was seized by gunmen yesterday.

Bracken was following bloody tracks on a sidewalk in the Miramonte residential district and encountered a wounded guerrilla who then shot him, according to the official version, which could not be confirmed independently.

There were conflicting reports tonight about the shooting. Witnesses told Salvadoran journalists that Bracken was killed in a shootout between a group of guerrillas and the National Police. No guerrilla group has claimed responsibility for the abduction of Machon or the killing of Bracken. Salvadoran leftists have charged that U.S. "mercenaris" assist the National Police, and two months ago claimed to have killed one American, who was unidentified.

Bracken is the fifth American victim of the recent wave of violence in El Salvador. On Dec. 3, three nuns and a Catholic volunteer worker, all U.S. citizens, were slain, leading to a U.S. investigation and the suspension of all military and economic aid to El Salvador.

The State Department, citing the restructuring of the Salvadoran government, today announced that it is resuming the economic assistance program but continuing the ban on military aid.

In another development, witnesses and authorities reported that a large bomb caused extensive damage to the offices of the Bank of America, the second attack against that bank this year.