By SEAN DOOLEY, JENNER SMITH and ALEXA VALIENTE

The best friend of a Florida man in prison for killing his parents before throwing a house party said he took a selfie with him to remember him by after learning about the murders.

"[I took the selfie] 'cause I knew it was going to be the last time I ever [saw] him," Michael Mandell, 20, told ABC News' "20/20."

In July 2011, authorities said, Tyler Hadley, then 17, killed his parents, Mary Jo and Blake Hadley, at their home in Port St. Lucie, Fla. Hadley then spent three hours cleaning up the blood before having a party while his parents' bodies remained in their bedroom.

During the party, Hadley told Mandell, then 17, about the murders.

Mandell noticed blood by a computer desk and by the master bedroom, he said, but initially he didn't believe Hadley was telling the truth.

"I actually thought he went that far out to play a prank on me, that he actually put fake blood on the ground," Mandell said. "Anything but what it is, you know."

Yet, even after he went on his own to the master bedroom and discovered Blake Hadley's leg against the door, Mandell remained at the party. The two friends even took that selfie together.

"When I took the picture, I [had] already seen 'em, seen the bodies," Mandell said. "I knew I was gonna call the cops on him when I took that picture."

(Port Saint Lucie Police)

While he has received criticism for taking the photo when he already knew Hadley's parents were dead, Mandell said that may not be relevant.

"Look at the picture. Am I smiling? Is Tyler smiling?" Mandell asked. "You know, it's a very sad picture. I don't look happy at all."

In the early hours of the next morning, Mandell went home and called Crime Stoppers to tell them Hadley killed his parents.

Hadley is now 20 and serving two life sentences without the possibility of parole.

"Like I said, still, to this day, I'll never see Tyler as a murderer. He is my … longtime childhood friend," Mandell said. "And that's all I'm ever going to see him as, for the rest of my life."

Tune in for the full story on ABC News' "20/20? on Friday, April 4, at 10 p.m. ET.