Hundreds of mourners turned out in Denver for a memorial service on Wednesday night for those killed in the school shooting in nearby Littleton.
Denver Mayor Wellington Webb told the crowd that he had no "magic answers" to stopping youth violence.
Several hundred students from around the Denver area gathered at a park near the Columbine High School on Wednesday, many with their parents.
Bouquets, balloons and condolence cards were scattered around the grounds.
Meanwhile the police said they believed that Eric Harris, who just turned 18, and Dylan Klebold, 17, were alone when they walked into the suburban high school on Tuesday and shot at students and teachers.
The attack left 15 dead, including the two assailants.
However the Jefferson County Sheriff said the elaborate nature of the plan prompted suspicion that others may have known about it.
"Well we've got a full investigation to do here we've got hundreds of kids who had brief interviews yesterday (Tuesday) we've got to go back an re-interview these kids to see who has seen what - to see who has seen what guns and what peoples hands. We know that these two are probably involved - the ones that are deceased we are uncertain if we've got other members in there. I have concerns because we've had tremendous amount of explosive devices in there, whether two people could carry that all that stuff in there. The other scenario could be that maybe some of that stuff was hidden in the schools.
SUPER CAPTION: Jefferson County Sheriff John P. Stone
District Attorney Dave Thomas outlined to the media what kind of investigation will unfold over the coming weeks.
"The investigation will be ongoing it will be - it will not be completed today (Wednesday) or even tomorrow (Thursday). This will be a, I believe a long and very extensive investigation. I will probably not be able to go into extensive detail about that investigation but it will include any one else who had knowledge or assisted in any way with what transpired at Columbine High School yesterday (Tuesday). There will be extensive interviews - we're in that process right now."
SUPER CAPTION: District Attorney Dave Thomas
On Wednesday evening gospel music filled the air in the Denver area as mourners took part in a vigil for the victims of Tuesday's school shooting.
Some of those who attended the vigil were still uncertain about the fate of friends at Columbine High School.
"I had a couple of friends from Columbine and I've heard from all but one. So I have no clue how he's doing if he's alive or not. It's really hard to deal with right now."
SUPER CAPTION: Robert Alba, Student
Others who attended the vigil said more such gatherings needed to be held to help the community cope.
"I believe it's taken away a lot of pain kind of helped me out with my overall feelings of the situation. But I think there's still a lot of stuff to be taken care of and I think that the community needs to be more involved in this sort of thing, not just this one get together isn't going to solve everything.
SUPER CAPTION: Ryan Swartz, Student,
Community and religious leaders spoke to the hundreds of mourners.
One speaker said it's time for Americans to say no to hate, violence and guns.
However Denver Mayor Wellington Webb said he has no "magic answers" to stopping violence among young people.
Other speakers said violence is everywhere -- in music, media and movies -- and that's devalued human life in the eyes of young people.
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