Fernando L. Ribas-Dominicci liked to go fast.

A pilot since college, the Air Force captain "really liked to fly," his brother Jose said yesterday. "He loved velocity."

On Monday, Ribas-Dominicci, 33, was at the controls of one of the 18 jet fighter-bombers sent from Britain to attack Libya. Seated directly behind him was his bombardier and navigator, Capt. Paul F. Lorence, 31, a classical pianist.

Their plane remained unaccounted for yesterday. U.S. military officials said the jet and its two-man crew were believed lost at sea.

Lorence, who grew up in San Francisco, learned to fly when he was 17, according to his stepfather, Richard J. Kruger. Lorence met his British-born wife, Dianne, while stationed at the Royal Air Force base at Lakenheath, England, Kruger said. The couple, married 18 months, has an 8-month-old son, Peter.

Ribas-Dominicci met his Mexican wife, Blanca Linda, while stationed in Texas, and they have a 4-year-old son, also named Fernando.

According to Jose Ribas-Dominicci, his brother learned to fly while in college, and when he wasn't going fast in airplanes, he enjoyed the speed of powerboats.

Both fliers were assigned to the Air Force's 48th Tactical Fighter Wing based in Lakenheath.

The Defense Department reported yesterday that despite an extensive search in the area where their F111 was believed to have gone down, no wreckage had been recovered and searchers had detected no signals from the homing beacons or lights that the fliers could have activated if they had ejected safely.

Ribas-Dominicci had been in the Air Force for eight to 10 years, his brother said, and had spent the last two in England. Lorence rejoined after college, eight years ago, and had been in England the last four years.

Relatives and friends were not optimistic yesterday about the fate of the men.

"We are very sorry," said Jose Ribas-Dominicci. "The news is nothing good."

He said he had spoken with his brother's wife, Blanca Linda, in England and she said that pilots who flew the mission with her husband reported seeing his plane going down before they got to Libya. Jose Ribas-Dominicci said it was unclear from what the pilots said whether his brother's plane had been shot down or was having mechanical trouble.

"We know exactly what happened," Jose Ribas-Dominicci said, "because the pilots said they saw his plane going down. But it was too dark -- they don't know if they ejected."

Fernando Ribas-Dominicci, the youngest of six sons of a furniture-store owner, grew up in the central Puerto Rican hill town of Utuado and graduated from the University of Puerto Rico with a degree in civil engineering, his brother said. Although he liked to go fast, "he was not a crazy, mixed-up kid," his brother said. "He was very serious. He was a very quiet person, a very polite person, a Catholic person."

Lorence was described by his stepfather as "quiet, unassuming," but assertive when the occasion called for it. He was "a natural leader," Kruger said, adding that Lorence's passion mirrored that of his maternal grandfather, a pilot in Britain's Royal Flying Corps during World War I.

Lorence enlisted in the Air Force out of high school, Kruger said, and after four years as a nuclear weapons specialist he returned to college, earning a degree in history from San Francisco State University. He rejoined the Air Force in 1978.

He was working toward a masters degree in international relations at Cambridge, Kruger said, and expected to make the Air Force his career. His home in England housed a sizable art collection and a piano, his stepfather said.