A 35-year-old man died yesterday afternoon in D.C. police custody, shortly after an off-duty officer fought with him and subdued him during a disturbance at a Northwest bank, police said.

Police said Officer Kevin W. Rachlin had entered the American Security Bank branch at 3401 Connecticut Ave. NW about 12:50 p.m. when he was asked by bank employes to remove a man who was harassing bank customers and employes.

Capt. William White III, a police spokesman, said that as Rachlin was escorting the man from the premises, the two became involved in a struggle and bank employes called police.

With the assistance of a citizen, Rachlin was able to subdue the man on the sidewalk outside the bank, and after other officers arrived, the man was handcuffed and placed in a transport car, White said.

When the car arrived at the 2nd District headquarters at 3320 Idaho Ave. NW, White said, officers discovered the man was unconscious.

The man, whose name was being withheld until relatives could be notified, was taken by fire department ambulance to Georgetown University Hospital, where he was pronounced dead at 2:20 p.m., White said.

White said the cause of death is officially undetermined but that a preliminary autopsy conducted by the D.C. medical examiner's office "indicated that there was no significant trauma to the body, no major injuries or broken bones."

Sources said the autopsy showed evidence of illicit drugs and alcohol that may have contributed to the death. White said investigators are awaiting a toxicology report as well as additional autopsy results before commenting further on the death.

Capt. Larry D. Soulsby, D.C. homicide branch commander, said the dead man was "apparently a person known to be a street individual up there" and that the struggle with Rachlin lasted 15 to 20 minutes. White said the man had caused a disturbance at the bank's outdoor automatic teller machine before entering the bank.

Franklin C. Salisbury Jr., the citizen who assisted Rachlin, said in a telephone interview last night that he was walking down the sidewalk when he noticed the disturbance outside the bank. He said the officer identified himself as a policeman, asked for help "and I assisted him, which is something any citizen would do."

He said Rachlin, a three-year member of the force who is assigned to the 7th District, remains on full duty.Staff writer John Ward Anderson contributed to this report.