For three decades, the FBI failed to disclose it had hundreds of hours of tape recordings and other evidence that could help convict former Ku Klux Klansmen in the 1963 church bombing that killed four black girls.

When that information finally came out in court in recent weeks and clinched the conviction of ex-Klansman Thomas Blanton Jr. on Tuesday, Alabama prosecutors who first broke open the long-dormant case in the 1970s were outraged.

"That is the most stunning revelation of this thing," said former Alabama Assistant Attorney General John Yung, who in 1977 helped send ex-Klansman Robert "Dynamite Bob" Chambliss to prison for the bombing. "I think it's shocking that someone sat on that evidence all those years."

Among the FBI evidence that had remained secret since the mid-1960s: secret tape recordings of Blanton talking about planning "the bomb" and the testimony of Mitchell Burns, a former Klansman and paid FBI informant who recorded other conversations.

"What excuse can the FBI have for allowing Mr. Blanton to go free for 24 years with this smoking-gun evidence hidden in its files?" former Alabama Attorney General Bill Baxley wrote in a commentary Thursday in The New York Times. "How can the FBI justify this to the families of four precious girls?"

Baxley reopened the church bombing probe in the 1970s and convicted Chambliss.

FBI spokesman Craig Dahle said there was no easy answer for the agency's failure to hand over everything it had years earlier. But he denied the FBI deliberately delayed justice.

"I think it is wrong to assert there was any effort to block anything," Dahle said.

Intentionally or not, then-FBI Director J. Edgar Hoover prevented a quick trial in 1965 when he concluded that Birmingham's racial climate meant a guilty verdict was highly unlikely. Juries in Birmingham were all white at the time, and the police force included Klan sympathizers.

"It was just a different time," Dahle said.

Also, he said, the FBI has a policy against revealing the identity of confidential informants like Burns unless they agree to go public. And agents may have been leery about sharing the information with state and local police forces, which were known to have Klan sympathizers.

As for the tapes secretly made in Blanton's kitchen in 1964, computers were used to enhance the murky recordings and help decipher the voices for the jury at his trial -- technology unavailable in the 1960s and extremely time-consuming in the 1970s when Baxley was pursuing the church bombers.

The tapes were only "moderately intelligible" without enhancement, said Anthony Pellicano of Forensic Audio Laboratory, the Los Angeles company that performed the work.

But the FBI did not let Baxley know the tapes even existed in the 1970s. It wasn't until the latest investigation began that Baxley found out about the tapes and the informant. Yung said he learned of the long-secret evidence as the Blanton trial unfolded.

"They denied having any more evidence than what they gave us, and it was hard enough getting what we got," Yung said.

With Hoover running the FBI, the case was closed in 1968 without any charges. The file of FBI evidence against Chambliss, Blanton and others sat dormant.

Hoover died in 1972, but his successors also left the tapes and other evidence in storage. Finally, Rob Langford, the agent in charge of the Birmingham FBI office, decided to reopen the church bombing probe after meeting with black ministers in 1993.

FBI agents spent months going through the old evidence and found the tapes and written reports compiled by Burns. The tapes went from storage to experts who enhanced their quality and then to prosecutors. U.S. Attorney Doug Jones was deputized to handle the state case, and Burns was brought in to testify before the grand jury that indicted Blanton last year.

"No matter what J. Edgar Hoover said and no matter what his reasons, the FBI and the agents that were on the ground ... did an absolutely incredible job," Jones said after the verdict.

Baxley, in his op-ed piece in the Times, said "rank-and-file FBI agents working with us were conscientious and championed our cause." Without naming anyone, he said his disgust was with "those in higher places who did nothing."