The commuter airplane that made a forced landing south of Dulles International Airport last week may have lost power in its engines because the crew members failed to prevent ice from forming in the engines, federal safety investigators said yesterday.

Investigators were focusing on whether the pilot and copilot of American Eagle Flight 3464 knew to take federally recommended steps to prevent ice from disrupting airflow into the plane's two turbine engines, said National Transportation Safety Board spokesman Michael Benson.

The airplane landed upright on a grassy field about 1 1/2 miles south of Dulles after losing power in both engines, Benson said.

One passenger was hospitalized until Sunday.

The pilot, copilot and five other passengers were treated for minor injuries.

One safety expert who examined the 19-seat Swearingen Metro II turboprop Thursday night, after the 7:43 p.m. emergency landing, noted the chilly temperatures and said "ice was visible at the base of the fuselage and underneath the wings."

Dulles reported 35 degrees at 8 p.m. that night.

The airplane had been flying from Newark to Dulles at an altitude of 6,000 to 7,000 feet, where certain combinations of moisture and cold can produce ice, one federal aviation expert said.

Power loss can occur if ice forms, breaks off and enters the engine air inlets, disrupting airflow, the expert said.

The focus of the investigation "is on whether the pilots followed proper procedure, based on what was available in the flight operating manual for that aircraft," Benson said.

"We'll have to look at the manual," he added.

Two devices used to prevent engine icing were found switched off when the aircraft was examined, said an aviation expert familiar with the investigation.

One device was the de-icing system that directs hot air into the Garrett TPE 331 engines.

The other is an engine ignition system, which prevents ice formation if operated continuously.

The safety board recommended in 1985 that the aircraft's manufacturer warn pilots to leave the ignition switch in the continuous operation position while flying through or out of icing conditions, rather than turning off the ignition as they would under normal flight conditions.

That recommendation came after a similar incident involving a Swearingen Metro II that lost power in both engines as it approached the runway at the Greater Cincinnati International Airport in Covington, Ky., on Jan. 8, 1985.

The Federal Aviation Administration adopted a recommendation to require the warnings.

The manufacturer, Fairchild Industries, told the safety board in January 1986 that it had revised the flight manual to include the warning:

"Engine heat and continuous ignition . . . must be used after leaving icing conditions until the pilot is confident that any significant residual ice on propellers, spinners or inlet lips will not be shed into the engine intakes."

In interviews with investigators, the pilots "have indicated that they were following procedures . . . . They didn't feel there was an ice buildup," Benson said.

Investigators planned to examine a sample of the fuel carried by the American Eagle aircraft, but said yesterday that it appeared unlikely that the airplane ran out of fuel.

The airplane took off from Newark with 1,300 pounds of fuel, and was expected to burn 750 pounds en route.

One investigator said that the team found pools of fuel at the scene.

After Air Florida Flight 90 crashed into the 14th Street Bridge on Jan. 13, 1982, killing 78 people, the safety board faulted the pilots for failing to use engine anti-ice devices.

That failure produced misleading power readings that caused the throttles to be set too low for takeoff.