A Fort Belvoir soldier, accused of last Thursday's slaying of a TWA employe in a National Airport restroom, claimed to have been under the influence of amphetamines at the time, according to testimony yesterday in U.S. Magistrate's Court in Alexandria.

Magistrate W. Harris Grimsley ordered the case of Pvt. Joseph Patrick Shay, 18, bound over to a grand jury after hearing testimony from FBI agent Jesse Loftin, the only witness called at the 45-minute hearing.

Courtroom spectators gasped as Loftin testified that airline ticket agent Michael Papa had been stabbed 25 times with a five-inch hunting knife during the incident. Loftin quoted Shay as saying, "I stabbed a man," and said Shay acknowledged that the knife was his.

Shay, who sat motionless during the hearing, told TWA service agent Tom Payne immediately after the attack that he was "high" after having taken "black beauties," or speed, both nicknames for amphetamines, Loftin said.

According to the agent's testimony, the mortally wounded Papa told medical attendants that a man had tried to rob him. But under cross-examination by court-appointed defense lawyer Stephen Picard, Loftin acknowledged that Shay also had claimed someone had tried to rob him.

Loftin said that authorities in El Paso, Tex, where Shay lived before joining the Army, reported that Shay had no prior criminal or narcotics record.