Authorities in upstate NY wondered how a man under GPS monitoring for child pornography possession was able to deactivate the monitor, enabling him to carjack a woman and her 10-year-old daughter, raping the girl and then killing the mother, last week. But now they know how David Renz managed to elude police detection: He successfully dismantled the ankle monitor and put it back together in an minute, exploiting a flaw with the monitoring company.

According to Syracuse.com, at 7:08 p.m. last Thursday, Renz "took apart his GPS ankle monitor, removed it from his ankle, reassembled it and set it down, off of his body" in less than 60 seconds.:

At 7:09 p.m., Renz was a free man. When Renz took the bracelet apart, a company in Colorado that makes the electronic monitors received an alert. But the company, BI Inc., didn’t notify the court officers in Syracuse who are responsible for making sure that Renz stayed away from children and obeyed a 9 p.m. curfew.

In the past, court officers were getting alerts telling them the bracelets had been tampered with when they’d simply been knocked against bathtubs or walls, momentarily interrupting the signal, [Chief U.S. Probation Officer Matt] Brown said.

Because of this, they asked BI not to notify them when a bracelet’s signal was interrupted for a short period of time, he said. Brown wouldn’t say exactly how long that window is, but it is more than a minute.

Brown said, "We finally figured out that the circuit can be interrupted because of something as simple and innocent as that," and apparently BI has admitted that this has happened before. Now the U.S. Office of Probation and Pretrial Services is reviewing its contract with BI.

Authorities say that Renz, 27, carjacked elementary school librarian Lori Bresnahan, 47, as she was leaving a gymnastics class at a mall in the Syracuse suburb of Clay with her 10-year-old daughter around 8:30 p.m. last Thursday night. He allegedly tied up the woman while he raped the young girl in a deserted part of the parking lot. The girl escaped as Renz was stabbing Bresnahan to death. Police eventually found him in a wooded area near the parking lot.

Renz was facing charges related to child pornography possession—he admitted to downloading about 100 gigabytes of child porn, made up of more than 500 videos and more than 3,000 images. He had a 9 p.m. curfew and was told to stay away from children.

Police still think the attack was random, though Renz and Bresnahan overlapped for over two years while he was a student at SUNY Oswego and she worked as an instructor there.