Omaha police released this undated photo on Sept. 4. Omaha Police Department Mugshot via SF Gate A convicted robber in Nebraska allegedly shot four people in the head weeks after finishing a decade-long prison stint that began when he was a teenager, the Omaha World-Herald reports.

Nikko Jenkins, 26, made "incriminating statements" about the shooting spree to investigators, according to the World-Herald. There's no clear motive for the killings. All but one of the victims had no connection to Jenkins, the World-Herald reported.

Jenkins reportedly behaved violently and erratically during his decade in prison.

He got written up for assaulting a guard, making threats, trying to escape, and creating a weapon out of a toilet brush, the World-Herald reported.

But he only served 10 and a half years in prison. That's just half the 21-year sentence he got for two carjackings and the assaults that happened when he was in prison, according to the World-Herald.

Nebraska law gives inmates "good-time" credit of a day for every day they serve. While he lost 17 and a half months of his "good time" credit, a judge gave him about 17 months of credit for time served in a county jail.

Jenkins' early release obviously calls into question the wisdom of "good-time" credit. It also calls into question the notion that prison has any power at all to rehabilitate criminals. Jenkins got locked up as a relatively young teenager and apparently just got more violent.

The murder of 33-year-old Andrew Kruger is particularly horrifying. Kruger, a wife and mother of three, was on her way home from work right before she died, WOWT.com reported.

Authorities believe her car pulled up behind Jenkins. Police believe Jenkins then pulled her out of her car and shot her in the head, according to WOWT.

The other victims were Juan Uribe-Pena, 26, Jorge C. Cajiga-Ruiz, 29, and 22-year-old Curtis Bradford. Bradford was the one victim who knew Jenkins, according to the World-Herald. The two met in prison.