SAN FRANCISCO, June 5 — Opening a new chapter in a case that the F.B.I. has been trying to close for years, a federal appeals court here ruled today that a sharpshooter for the agency could be tried on state charges of manslaughter in the death of a woman during a standoff in 1992 in Ruby Ridge, Idaho.
The 6-to-5 decision by the full United States Court of Appeals for the Ninth Circuit reverses a ruling by a three-judge panel from the same court. The ruling is the latest in a series of setbacks for the bureau, including its belated disclosure of numerous documents in the Oklahoma City bombing case that led Attorney General John Ashcroft to postpone the execution of the convicted bomber Timothy J. McVeigh. Last week, an F.B.I. agent, Robert P. Hanssen, pleaded not guilty to spy charges. He was accused of giving secrets to the Soviet Union and Russia in return for $1.4 million.
Since the 11-day siege in August 1992 at Ruby Ridge, the case has been held up by anti-government groups as a benchmark example of the federal government's excessive use of force. Three people died in the standoff at the home of Randy Weaver, a white supremacist, including Mr. Weaver's wife, Vicki, the couple's 14-year-old son, Samuel, and a deputy United States marshal, William Degan.
The standoff began when federal agents tried to arrest Mr. Weaver at his cabin for failing to answer illegal gun charges.
During the standoff, the sharpshooter, Lon T. Horiuchi, an F.B.I. agent who still works for the bureau, shot and killed Mr. Weaver's wife and wounded a family friend, Kevin Harris. Witnesses have said the sharpshooter fired as Mrs. Weaver, 42, held open the cabin door, her 10- month-old baby in her arms, to let Randy Weaver, their daughter and Mr. Harris inside.
Mr. Horiuchi has said he did not see Mrs. Weaver when he fired at Mr. Harris, who was armed as he ducked into the cabin.
Both Mr. Harris and Mr. Weaver were acquitted of murder, conspiracy and other federal charges. Mr. Weaver, who became a hero to anti- government organizations, was convicted of failing to appear for trial on the firearms charge that brought federal agents to his cabin in the first place.
After an extensive two-year investigation, the federal government declined to prosecute Mr. Horiuchi.
But in 1997, an Idaho prosecutor filed charges of involuntary manslaughter against Mr. Horiuchi, accusing him of excessive use of force. The felony charges carry a maximum penalty of 10 years in prison.
Mr. Horiuchi succeeded in having the case transferred to Federal District Court in Boise, Idaho, where the charge was dismissed on May 14, 1998.
In June 2000, a three-judge panel from the Ninth Circuit dismissed an appeal of the transfer, ruling that the Idaho prosecutor could not try a federal agent on manslaughter charges for "actions taken in pursuit of his duties as a federal law enforcement officer."
But today the court, while still clearly troubled by the notion of trying the agent, disagreed with arguments that it did not matter whether Mrs. Weaver's death was the result of excessive force.
"When federal officers violate the Constitution, either through malice or excessive zeal, they can be held accountable for violating the state's criminal laws," Judge Alex Kozinski wrote for the divided appeals court.
The majority agreed with arguments that immunity could not be granted until a trial was held to determine whether Mr. Horiuchi acted unlawfully. Those arguments were presented to the court in December by Ramsey Clark, the former attorney general under President Lyndon B. Johnson who was representing Boundary County, where the prosecution was sought.
"When federal law enforcement agents carry out their responsibilities, they can cause destruction of property, loss of freedom, and as in this case, loss of life — all which might violate the state's criminal laws," Judge Kozinski wrote.
But the dissent in the close ruling said the majority was "dissecting the mistakes" made in a tumultuous situation and said the majority's opinion was a "grave disservice" to F.B.I. agents.
The Justice Department said it had not yet decided how to respond to the federal appeals court decision on Ruby Ridge. "We need to review the court's decision today to determine what our next step will be," said Charles Miller, a Justice Department spokesman.
Louis J. Freeh, who is leaving as F.B.I. director, said he was disappointed with the ruling and said the agency stood behind Mr. Horiuchi.
Mr. Freeh told The Associated Press, "We continue to believe strongly agent Horiuchi met the legal standard that protects law enforcement officers when they carry out their duties, even when the consequence in hindsight is regrettable."
Lawyers for Boundary County called the decision a "significant victory for individual rights and states rights, adding that "it puts another nail in the open coffin" of the F.B.I."