Authorities decided soon after the Columbine High School massacre to withhold a document showing deputies knew one of the killers had been accused two years earlier of making death threats and building pipe bombs, according to a grand jury report released Thursday.

The grand jury also said it was "troubled" by still-missing documents in what remains the deadliest school attack in U.S. history. But it did not hand up any indictments, closing at least the third investigation that has elected not to place any blame for the slaughter of 13 people by suicidal teenagers Eric Harris and Dylan Klebold.

Victims' families said the report confirmed their suspicions that the Jefferson County Sheriff's Office covered up mistakes that could have led authorities to the killers as much as two years before the attack.

"Clearly, Columbine never should have happened, and they don't want the public to know that," said Brian Rohrbough, one of several family members who met Thursday with Attorney General Ken Salazar. Rohrbough's son, Daniel, was among the students slain on April 19, 1999.

The grand jury said it did not hand up any indictments because all the witnesses said they know nothing about the missing sheriff's records. The records involved a draft search warrant for Harris's house a year before the attack.

Salazar took the case to the grand jury after the sheriff asked him to investigate last fall. The department would not comment on the report.

The grand jury report said the draft affidavit, never submitted to a judge, was the subject of "a private meeting of high-ranking Jefferson County officials and law enforcement officials" called a few days after the attack to discuss the document and its "potential liabilities."

As a result of that meeting, the sheriff's office decided against discussing the affidavit in an April 30, 1999, news conference. A state district judge ordered the affidavit's release in 2001.