The student who shot dead at least 30 people at Virginia Tech was admitted to a mental health unit in late 2005, police have revealed.
Cho Seung-hui was sent for evaluation after two female students made complaints against him, they said.
Cho had also sent "photographs, video and writings" to NBC News, which have been turned over to the FBI for examination, police said.
MSNBC said the material was sent between the first and second shootings.
A total of 32 people died in shootings at two locations on the Virginia Tech campus on Monday.
"This may be a very new, critical component of this investigation," said Steve Flaherty, superintendent of Virginia State Police.
"We're in the process right now of attempting to analyze and evaluate its worth," he said at a news conference.
The complaints about Cho were made in November and December 2005, around the time Cho's English teachers raised concerns over his writing and general behaviour.
Authorities have not yet linked the 23-year old to any of those he killed.
Two people were killed at the West Ambler Johnston Hall, a university dormitory, before Cho killed 30 others, plus himself, at the Norris Hall complex across campus some two hours later.
Police say the same gun was used at both locations but have not definitely proved that Cho was at West Ambler Johnston at the time of the shootings there.
In the aftermath of the shootings, teachers and fellow students have spoken of Cho's moods, violent writings and unpredictable behaviour.
One of his roommates, Karan Grewal, told the BBC that Cho rarely spoke to those with whom he shared a three-room apartment.
"I figured he was pretty lonely, but not that he was angry in any way or he was capable of what he did."
Two separate complaints about Cho's behaviour were lodged in late 2005, university police chief Wendell Flinchum told a news conference.
But he said the two women who made the complaints were not among Cho's shooting victims.
However, Mr Flinchum did reveal that Cho was well known both to campus authorities and local law enforcement agencies.
"I'm not saying they were threats, I'm saying they were annoying. That's the way the victims characterised them, as annoying messages," he added.
In the first instance, Cho reportedly telephoned a female student and made direct contact with her.
Police spoke to Cho after she lodged a complaint, they said.
In a similar incident a month later, in December 2005, Cho reportedly made contact with another female student through instant messaging, leading to her complaint.
He was referred to a mental health unit outside the Virginia Tech campus on 13 December for evaluation amid concerns he was feeling suicidal, police said.
Private medical records from the mental health facility remain confidential, but Cho was referred back to university authorities for counselling after his assessment and had no further contact with campus police.
Around the same time, at least two of Cho's English teachers voiced their concerns over his behaviour and the tone of his work.
Cho's writing was moody and often involved themes of violence and death that alarmed Lucinda Roy, who was at that time head of the English department at Virginia Tech.
She removed Cho from regular classes and tutored him one-on-one after a complaint from another teacher.
"He was quite a gifted student in some ways, but he seemed to be very lonely and depressed," she said.