On Saturday, Yvonne Mollino, a 17-year-old senior at Southampton High School on Long Island, N.Y., her parents and younger sister flew in a small plane to the District of Columbia so that Yvonne could visit George Washington University before deciding which college she would attend next year.

Her father, an experienced pilot, flew the family to National Airport in the morning and the four spent an apparently uneventful day here. About 4:30 p.m., the family boarded their rented Cessna 206, a single-engine plane, and left National for a Long Island airfield.

They never arrived. Thirty miles south of Long Island, pilot Joseph Mollino, 41, told Kennedy Airport that his fuel gauge read "zero," airport and federal aviation officials said yesterday. Minutes later, the plane crashed at sea three miles from land, officials said.

A 13 1/2-hour search by Coast Guard and Nassau County marine police teams in boats and helicopters was suspended early yesterday morning after no survivors and no debris were found, said Petty Officer Jerry Snyder, a Coast Guard spokesman. The plane apparently crashed in 50 feet of water near Long Island's Tobay Beach, Snyder said.

Teams did find a small oil slick in the middle of the 10-mile search area, but Snyder said he was uncertain that it came from the plane.

Believed dead were Joseph Mollino, a lawyer with an office in the affluent village of Southampton; his wife, Elfie Gustafsson; and their daughters, Yvonne and Chrissie, 14.

Although officials first refused to release the Mollinos' names, word of their deaths quickly swept their Southampton Township neighborhood of North Sea yesterday.

Cheryl Raffel, a close friend of Yvonne Mollino, described her classmate as "an attractive girl whose personality made her."

"She was always bubbly," said Raffel. Yvonne was an excellent student, editor of the high school yearbook and was certain to win a spot on the cheerleading squad--as she had for the past three years, Raffel said.

Yvonne told Raffel on Friday that she wanted to attend Bucknell University or a New York college next year, but considered GWU a "backup" in case she was not admitted there, Raffel said.

A GWU spokesman said yesterday that he did not know whether Mollino had an admission interview at the school.

Michael T. Kuzenko, a senior air safety investigator with the National Transportation Safety Board in New York, would not speculate on the reasons for the crash. An investigation is planned, he said.

Jack Rera said Joseph Mollino, his nephew, had been flying small aircraft for at least 10 years, owned a two-seat plane and was a part-time flight instructor. "He didn't run out of gas," said Rera. "He was too experienced a pilot for me to believe that."