On Monday, it was revealed Castillo purchased a shotgun and rifle months before the August 2006 shooting.
One witnessed recalled Castillo practicing with his weapons at a firing range in Burlington with his father.
A bomb expert with the Durham Sheriff's office also detailed the homemade explosives found in Castillo's car and home.
The defense started its case Monday afternoon, calling one of the first Orange County investigators at the scene to the stand.
He told them he had Castillo immediately committed, because he was making suicidal statements.
"I recall that he said he wanted to die," the witness said. "I don't recall him saying he wanted to kill me or himself. I just recall him saying he wanted to die."
A psychiatrist also testified to Castillo's mental state and said he suffered from delusions. She said he told her he planned a suicide attempt to coincide with the Columbine massacre anniversary.
On Tuesday, the defense will continue making arguments as to why Castillo was not of sound mind when he opened fire on Orange High School.