A 16-year-old Gaithersburg youth has been convicted in Juvenile Court of manslaughter in connection with a head-on auto accident in rural Montgomery County last November that killed four members of a Gaithersburg family.

Court sources identified the youth yesterday as Florin Dan Iacob of Goshen Court. They said Judge Douglas H. Moore ruled that Iacob was "involved," the Juvenile Court terminology for guilty, in four counts of manslaughter and 10 traffic violations, including reckless driving, crossing the center line and speed greater than reasonable, in the two-car crash that killed four members of the Alan Lighthall family.

Judge Moore, who heard nine hours of testimony in a closed hearing last Thursday, found that Iacob was "not involved" on a charge of participating in a speed contest, according to the sources.

A spokesman for the Montgomery County state's attorney said the judge may place the youth on probation or send him to one of several youth centers, such as a forestry camp, group home or training school, for an indefinite period, up to age 21.

Iacob's attorney, John T. Bell, said yesterday his client got a "fair and just trial."

According to police reports, the youth was driving south on Gaithersburg-Laytonsville Road (Rte. 124) last Nov. 15 about 6:45 p.m. when his late-model Mercedes swerved and crashed head-on into a northbound 1980 Pontiac Phoenix driven by Army Capt. Alan Lighthall..

Killed in the crash were Lighthall, 31; his wife, Jana Lynn, 27; their son, Nathan, 4, and daughter, Rebekah Lyn. 1.

The only survivor in the Lighthall car was 5-year-old Brian, who was riding in the back seat with his younger brother. Brian suffered multiple injuries.

Lighthall, a supply officer at the Walter Reed Army Clinic, who was scheduled to be transferred from Washington to Atlanta, reportedly was en route to a party in his honor in nearby Damascus at the time of the accident.