A 4-year-old boy who had wandered away from his home in the Mount Vernon area of Fairfax County died yesterday after being found under the water of a neighbor's swimming pool, in what county police described as a "tragic accident."

The boy, Timothy J. Mitchell, of 3713 Colonial Ave., was found about 3:30 p.m. in a pool at 3905 Colonial Ave., police said. They said he was taken first to Mount Vernon Hospital and then to Children's Hospital, where he was pronounced dead about 6:30 p.m.

County police spokesman Warren Carmichael said the boy apparently drowned, but the official cause of death could not be learned last night. Carmichael said an autopsy will be conducted.

Police said Laura L. Mitchell, the boy's mother, reported him missing about noon. They said she told them had had wandered off for no known reason about 10:30 a.m.

Police searched the neighborhood, an area of single-family houses on 3/4-acre lots, for several hours on foot and with a helicopter. While conducting a door-to-door canvass they asked the occupant of the house at 3905 Colonial Ave. to search his pool, police said.

Carmichael said police had been told earlier that the pool had been checked and that nothing had been found. This time, he said, officers found the boy in the water, which he described as murky and eight feet deep.

A man who answered the telephone at the house last night declined to comment on the incident.

Timothy's grandmother, Edwina Mitchell, described him last night as a "lovable, active little boy" who liked to play outside and would occasionally walk to neighbors' houses.

"He was very active, very friendly," she said.

The boy had three brothers, aged 11, 9 and 3.

Several neighbors on Colonial Avenue said they had joined in the search yesterday.

Arthur Ferreira said he noticed officers "constantly going up and down in the cruisers" and checked the manholes at a nearby construction site in an effort to locate the child.

Some of the neighhbors said the accident made them anxious for the safety of their own children.

"We have an 18-month-old who's learned to unlock the front door, so we're almost in the same situation," said Timothy Kelleher.

Dora Frye, another neighbor, said many families on the street have young children who often play together outside.

"You never dream that something would happen that close to home," she said. "You just never dream of what they'll get into."