Sacramento, Calif. -- It was a brazen, tightly organized attack. A gang of men and women wearing ski masks burst into a bank in Carmichael, Calif., on an April afternoon in 1975, killed a female customer and escaped in a series of getaway cars with $15,390. But the police found virtually no evidence and had no idea that it was the work of the radical Symbionese Liberation Army.
"It was downright irritating," said Terry Dyer, a Sacramento County sheriff's deputy who is now retired. "We had no proof of anything, no leads, no nothing."
But on Wednesday, Sara Jane Olson and three others were arrested and charged with first-degree murder in the case. The others are William Harris; his former wife, Emily Harris; and Michael Bortin. A warrant was issued for a fifth person, Jim Kilgore, but he has been a fugitive for decades.
The new charges were the results of years of on-again, off-again investigations, several task forces, thousands of hours of assistance from the Federal Bureau of Investigation, heavy pressure from the family of the slain woman, Myrna Opsahl, and from the Los Angeles district attorney's office.
The difference now, said Jan Scully, the Sacramento County district attorney, was that new corroborating evidence had been developed, replacing that initial void with what one person with knowledge of the case said was now an array of circumstantial and direct evidence, buttressed by the testimony of several crucial witnesses.
In describing the initial investigation, Mr. Dyer said the only things significant that the police knew in 1975 were how well organized the gang was, which he described as highly unusual, and the fact that the group included women, based on the voices that witnesses heard inside the bank.
Mr. Dyer, who has been consulted regularly on the case, said "I can tell you, there is a lot more we know now about that group and the crime because of a lot of work by a lot of people and these new methods."
In citing the new evidence, Ms. Scully singled out a new forensic technique used by the F.B.I. that had definitively allowed investigators to connect shotgun pellets found in the body of Ms. Opsahl, who had been shot in the abdomen at close range, with other shotgun shells later found in a Symbionese Liberation Army hideout.
That connection had not been possible before. Indeed, shotguns, which blast a cloud of small pellets rather than a single bullet that is imprinted with telltale marks, have long been notoriously difficult to connect to specific crimes.
One person close to the investigation said that would be critical in any trial, but he said many pieces of evidence would be used, demonstrating not just the guilt of those being charged but also the remarkable amount of time the government devoted to a crime that, even when it took place, attracted relatively little attention.
The evidence will include, he said, many fingerprints and palm prints connecting some of those accused with guns and other items used in the crime. In addition, the prosecutors will be able to rely on the testimony of at least three people who were reportedly directly involved in the crime.
The three, Patricia Hearst Shaw, Wendy Yoshimura and Steven Soliah, have all been granted immunity from prosecution in return for their testimony.
One concern that investigators have long had is that their best witness, Ms. Shaw, might not have been good enough on her own to obtain convictions.
Ms. Shaw was kidnapped by the radical group in 1974 in an effort to persuade her wealthy family to donate money to the poor. But she eventually joined the group and participated in a bank robbery and other crimes. She was prosecuted and went to prison after her capture in 1975. She was released after 18 months, then was pardoned by President Bill Clinton in 2001.
More important, she began cooperating with the police and wrote a book, "Every Secret Thing," in which she provided a detailed account of the bank robbery and the killing of Ms. Opsahl.
The book describes everything from the disguises the gang wore to the alarm some members expressed over the shooting.
Ms. Harris fired the gun directly against Ms. Opsahl's side, according to the book. The gun had accidentally fired twice when the group was rehearsing the robbery, and some members told Ms. Harris not to use it, the book said, but she insisted.
Prosecutors had for years been concerned about Ms. Shaw's testimony because of contentions that her book understated her own crimes and overstated those of others.
Ms. Shaw was scheduled to be a critical witness in a trial of Ms. Olson that was to have taken place in Los Angeles late last year on charges that she had plotted to bomb two police cars in 1975. She pleaded guilty to the charges and is scheduled to be sentenced on Friday.
Her guilty plea meant that Ms. Shaw would no longer have a trial run, so the prosecutors here decided to move.
"This case no longer stands or falls on her testimony," said one law enforcement official, adding that the prosecutors now have a devastating breadth of evidence.