The body of a boy believed to be 9 years old was found in a tributary of the Potomac River just south of the District of Columbia last night and U.S. Park Police said they were investigating the possibility that the boy had been struck with a baseball bat and fell or was knocked into the water.

U.S. Park Police Sgt. Tony Ferraro said detectives were interviewing two youths from the District, aged 12 and 13, in connection with the incident, which he said apparently occurred about 11 a.m.

Ferraro said the 9-year-old could not be positively identified immediately and an autopsy was scheduled to determine the cause of death.

His body was found about 10:20 p.m. in a Potomac tributary just north of the Woodrow Wilson Bridge, near the Oxon Hill Farm, which is operated by the National Park Service.

Early this morning, the Park Police issued a formal statement saying that they were notified last night that an assault had occurred near the farm. They said they were told that a witness had come forward who claimed to have seen the assault.

Earlier, Ferraro had said that one of the two youths being interviewed last night called authorities about 9 p.m. and said he wanted to look for a friend.

Ferraro said that according to an account given to police, the boy was struck with a baseball bat and fell or was knocked into the water, but he gave no further details.