The student who shot dead at least 30 people at Virginia Tech University 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.
The complaints were made in November and December 2005,around the time Cho's English teachers raised concerns over his writing and general behaviour.
Authorities have so far failed to link Cho to any of the students he killed.
The two women who complained about Cho's behaviour were not among the victims, officials said.
However, police told a news conference at the university that Cho was well known both to campus authorities and local law enforcement agencies.
In the aftermath of the shootings, teachers and fellow students have spoken of Cho's moods, violent writings and unpredictable behaviour.
Two separate complaints about Cho's behaviour were lodged in late 2005, police said.
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.