FBI spokesmen said yesterday they were awaiting the results of ballistics tests on the bullet fired through the window of U.S. Supreme Court Justice Harry A. Blackmun's third-floor Rosslyn apartment last week for possible clues as to who fired the shot.

An investigator who asked not to be identified said yesterday that preliminary ballistics tests indicated the bullet was shot from a great distance, most probably with a hand gun, and could have been fired randomly from the other side of the Potomac River.

"We haven't ruled anything in and we haven't ruled anything out," FBI spokesman Lane Bonner said yesterday, declining to confirm or deny the investigator's report. He added that the bureau has not connected the shooting to abortion opponents who have previously written threatening letters to Blackmun, the author of the Supreme Court's 1973 landmark decision legalizing abortion, Roe v. Wade.

In addition to investigating the shot fired at Blackmun's apartment, Bonner said the FBI is conducting a separate, and possibly unrelated, investigation into threatening letters sent last Thursday, the same day as the shooting, to Blackmun, Supreme Court Justice Lewis F. Powell and Sen. Alfonse D'Amato (R-N.Y.).

D'Amato aide Ed Martin said yesterday that the senator received an anonymous letter Feb. 28 from Buffalo that read: "Sir -- I do not like the way you are doing your job. One day I am going to see you and shoot your brains out. I am going to shoot you dead and I will be coming to your funeral."

Martin said, and an FBI source confirmed, that similar letters were sent to Powell and Blackmun that day from Buffalo, and that the FBI field office there is investigating them.

The letter appeared only to add to the confusion yesterday since D'Amato is a firm opponent of abortion, Martin said. Powell, the third recipient of a threatening letter from Buffalo, has written opinions expanding on abortion rights.

CBS News reported Monday that a source said trajectory tests showed the 9-mm bullet entered Blackmun's window at a slight downward angle, which could mean it was shot from the other side of the Potomac. The network also said preliminary tests showed the bullet, which left a baseball-size hole in the window, was nearly spent when it hit the glass and could have been fired from a handgun.

The incident became public Monday when Blackmun's office issued a brief statement. Blackmun, 76, had just left the room when the bullet entered the window shortly after 10 p.m. and reportedly showered glass fragments on his wife Dorothy. Neither was injured.