The two students who gunned down 12 classmates and a teacher at Columbine High School (search) bragged about making pipe bombs and said they were looking for a "ground zero" two years before the bloodbath -- and authorities knew it, the sheriff said Wednesday.

Authorities have said they knew about violent rants by Eric Harris (search) and Dylan Klebold (search) in 1998, a year before the shootings, and that a search warrant was not acted upon.

But Jefferson county Sheriff Ted Mink, who took office in July, said Wednesday that someone had called the sheriff's office a year earlier, in 1997, to tip off authorities about a Web site run by Harris.

A deputy investigated the tip and forwarded a report and printouts from the Web site to a sheriff's investigator in charge of computer-related crimes, Mink said.

The investigation of the Web site apparently stopped there, Mink said. The report was not found until last week, when a deputy came upon it as he was leafing through a training manual, the sheriff said.

On the Web site, Harris and Klebold described building pipe bombs from scratch.

"Now our only problem is to find the place that will be `ground zero,"' they say, according to the report.

Some families have been sharply critical of the sheriff's office and Mink's predecessor -- Sheriff John Stone -- for the handling of the Columbine investigation. They have accused authorities of ignoring or missing warning signs that Harris and Klebold planned to kill.

Sue Petrone, whose son, Daniel Rohrbough, was killed at Columbine, said she was astonished to see the year 1997 on the report.

"That's another chance that someone had to keep my son alive," she said.

Some details about the Web site were released in the months following the shootings, including nonspecific threats of violence and boasts about the teenagers' ability to build pipe bombs.

Mink said he had asked Colorado Attorney General Ken Salazar to investigate why the 1997 report had not been included in the hundreds of pages of documents that were reviewed as part of the probe into the massacre.

"The obvious implication is that the sheriff's office had some knowledge of the activities of Eric Harris and Dylan Klebold in the years previous to the Columbine shootings," Mink said. "It is difficult to believe this turn of events, and difficult to stand here and discuss them with you, but I feel it is the right thing to do."

Mink and Salazar met with families of the victims to discuss the report before it was released.