A prominent forensic psychologist has praised a tipster and police, saying they potentially prevented a mass killing after quickly responding to threats posted on a Facebook page suggesting similar crimes to the recent brutal shooting in Colorado.

Don Dutton - a UBC psychology professor who worked for the prosecution in the O.J. Simpson trial - applauded the tipster who notified police of "detailed and graphic" threats on Facebook as well as police who subsequently arrested North Vancouver's Ryan Lewis last week. Lewis, 32, appeared in court Wednesday, charged with uttering threats to cause death or bodily harm.

"It's typically after the fact that people will go back and say, 'Oh, he put all this stuff up on the Internet but we didn't pay any attention, we didn't take it seriously,'" Dutton said, referring to the 1989 Ecole Polytechnique massacre in Montreal and the 2007 Virginia Tech shootings. "So it makes a lot of sense to take it seriously."

RCMP said they were informed of the open death threats on Facebook on July 24. According to Cpl. Richard De Jong, the postings supported the July 20 shooting rampage at a midnight screening of The Dark Knight Rises at a movie theatre in suburban Denver, in which 12 people were killed and 58 others wounded. The threats involved committing similar acts in Vancouver and North Vancouver.

Accused Colorado shooter James Holmes is charged with multiple counts of first-degree murder and counts of attempted murder. Reportedly, his psychiatrist Lynne Fenton warned a so-called threat-assessment team at the University of Colorado, Denver, in early June of Holmes' odd behavior but no action was taken.

Police located the owner of the Facebook profile and on Aug. 1 arrested Lewis at a private residence in the lower Lonsdale area of North Vancouver, where he lives with his mom. Police seized his computer and the Facebook account was closed. Lewis has been released on numerous conditions, including curfews and not having access to the Internet.

Dutton called it "premature" to label it a copycat event, but said it was "not surprising at all somebody would pick up on this theme. In fact, mass killers in the past have picked up on prior mass killings as inspiration for what they did," he added, citing Columbine as an example.

"I'm sure there's a lot of disturbed people it resonated with in some particular way," Dutton said.

Dutton said while it's rare people who make such threats actually act upon them, experts still don't know what pushes them from making threats to carrying through with them.

"Nobody knows the answer to how they get from that point from angry rumination to what's called ideation where they've got definite plans to kill, and then to actually implementing the plan," Dutton said. "We know this much about them: they always isolate, they're sitting alone, they ruminate a lot. That's when they usually start buying the weapons so you can sort of see their death spiral deepening and deepening, but why it continues on for some people and not others, we just don't know."