A man in Washington, D.C., was expected to be in his home with his ankle monitor on, but that didn’t stop him from fatally shooting someone because the ankle monitor was actually on a prosthetic leg.

Quincy Green was supposed to be at his D.C.-area home on May 19, but instead, according to police, he shot and killed Dana Hamilton about 2:40 a.m., the Washington Post reports. Authorities said Green should have an alibi since he was at his house with an ankle monitor, but the device was actually put on a prosthetic leg — allowing him to take it off and use another.

“I don’t understand how someone could put this device on a prosthetic leg,” Sgt. Matthew Mahl, chairman of the D.C. police union, told the Washington Post. “It is frustrating for us as police officers to have one of our defendants released, especially when talking about dangerous crime like guns -- and then to know that the accountability for these defendants isn’t always up to par.”

Police said the GPA tracking showed Green still at his home during the time of the shooting, but he actually slipped off his leg and put on his spare to go out. Chris McDowell, director of communications for California-based Sentinel Services, told the Washington Post that “protocol for Green was absolutely not followed” since the technician put the device over a sock and didn’t realize his leg was a fake.

“We believe it was absolutely human error,” he said. Green was arrested on May 25 and charged with second-degree murder on Friday.

Read more at The Washington Post.