A confidential government memo says an FBI sniper who killed Vicki Weaver in a northern Idaho standoff won't be prosecuted because it's reasonable to believe he shot her to protect a government helicopter.

A confidential Justice Department report obtained by the Spokesman-Review newspaper said sniper Lon Horiuchi could be charged only if it could be shown that his fears for the safety of those in the helicopter were unreasonable and he went out of his way to use excessive force.Whether Horiuchi or other federal officers were to be charged in the death has been a major point of contention in the case.

White separatist Randy Weaver and his family and friend Kevin Harris went through a 1992 shootout with armed federal marshals, leading to an 11-day standoff at the Weavers' mountaintop cabin near Naples in northern Idaho.

Samuel Weaver, 14, was shot to death in the initial exchange along with William Degan, deputy U.S. marshal. Vicki Weaver, 42, was fatally wounded by a bullet fired by Horiuchi during the standoff.

Randy Weaver and Harris were charged with murder and other crimes, but a jury in Boise last year acquitted them of all the serious charges.

The memorandum also said there was not enough evidence to prove that Horiuchi's superiors meant to break the law or violate the Constitution when they wrote the deadly force rules that permitted agents to shoot any armed adult male seen in Weaver's yard during the standoff.

On the second day of the standoff, Horiuchi testified that he shot at armed adults in the yard after a man he thought was Weaver raised a rifle at a helicopter carrying federal agents.

A federal agent who was in the helicopter testified it never flew over the cabin, and an assault charge based on the allegation was thrown out by the judge at Weaver's trial.

But even if he was mistaken about the threat to the helicopter, "there is no evidence to prove that Horiuchi willfully used excessive force," said Deval Patrick, Civil Rights Division chief, in the memo stating no charges would be filed.

The decision was made despite a 542-page report by two dozen Justice Department investigators who said the department should consider charging FBI agents with violating Vicki Weaver's civil rights.

The report found that agents used unconstitutional deadly force rules in trying to arrest Weaver, a fugitive eluding trial on gun charges.

The Justice Department staffer who leaked the document said prosecutors didn't believe they could make a case because Horiuchi's fears were reasonable, if unfounded.

"The question is what Horiuchi believed," the official said. "He may have been mistaken, but that doesn't get us where we've got to go."