David J. Renz, the 29-year-old Cicero man charged with stabbing a school librarian to death and raping a 10-year-old girl, knew his way around technology.

He outsmarted the ankle bracelet that was supposed to keep the community safe while he was waiting to be tried on child pornography charges.

And when federal agents took his computer because they suspected there was child pornography on it in June, they had to go back to Renz in November and ask him for a key because information was encrypted.

Before Renz attacked 47-year-old Lori A. Bresnahan and a 10-year-old girl in the Great Northern Mall parking lot Thursday night, he deactivated the ankle bracelet he was fitted with when federal authorities released him from custody in January on child pornography charges.

The monitoring company didn’t detect anything until Renz had already committed the crimes and was in police custody.

Renz, who has a computer science degree from SUNY Oswego, deactivated his ankle bracelet at 7 p.m., according to Assistant U.S. Attorney Lisa Fletcher. His curfew required him to stay at his mother’s house on Lakeshore Road.

Police said Renz followed the child out of the CNY Gym Centre around 9 p.m. He tied up Bresnahan and raped the child in the mall parking lot, police said. Then he drove Bresnahan’s car to nearby Verplank Road in the town of Clay.

The child escaped while Renz was stabbing Bresnahan. Bresnahan died of the wounds at Upstate University Hospital.

During a hearing in federal court Friday afternoon, Assistant U.S. Attorney Lisa Fletcher said Renz deactivated his ankle bracelet.

The device is supposed to immediately notify probation officers that it has been tampered with, but Renz was able to somehow alter it so this didn’t happen. It wasn’t until 11:30 p.m. that the company, BI of Boulder, Colo., noticed that there had been no movement from Renz’s bracelet for hours and notified the U.S. Probation Office.

By that time, Bresnahan was dead, a 10-year-old had been brutally attacked and Renz was in police custody.

Chief U.S. Probation Officer Matthew Brown wouldn’t say specifically what happened in Renz’s case. He said the bracelets are supposed to immediately alert officers if tampering occurs. But there are ways trick the GPS system.

“We have since found there are ways to circumvent that without us being notified,” Brown said.

Renz had no criminal record before his January 9 arrest for possession of child pornography, said Executive Assistant U.S. Attorney John Duncan.

The complaint in that case shows that Renz was running a small computer lab in his North Syracuse apartment. He had four computers.

During his interview with FBI agents, he admitted to using one of those computers to store the child pornography. Renz said he had been viewing child pornography for six years.

On June 4, agents took the computer Renz said had child pornography on it. They attempted to search it June 6, but couldn’t because it was encrypted, according to the complaint filed against Renz in the federal child porn case.

FBI agents returned to Renz’s apartment in November and asked him for the key, which Renz gave them. They searched the computer files a month later and found more than 500 videos and 3,000 images that appeared to be child pornography, according to the complaint.

Of those, 13 videos and 254 images matched others in a child pornography database where the victims have been identified by the National Center for Missing and Exploited Children and are known to be under the age of 18, according to the complaint.

Two days after he was arrested, on Jan. 11, U.S. Magistrate Andrew T. Baxter, with the agreement of the U.S. Attorney’s Office, released Renz under certain conditions. Renz did not have to post any bail.

Baxter required Renz be at his mother’s home between 9 p.m. and 7 a.m. Renz could only use a computer at his job. Court papers say he lost his job at Wegmans, which he’d had for several years. And he had to wear an electronic monitor.

A company that provides the federal government with electronic monitors has one model: The ExacuTrack One. The company’s website boasts that the device is “lightweight, rugged, tamper-resistant.”

Renz graduated from East Syracuse-Minoa High School in 2001. He graduated from SUNY Oswego in 2011 with honors in computer science after first attending the University of Illinois at Champaign-Urbana and Onondaga Community College, according to officials at SUNY Oswego.

Renz’s family declined Friday to comment. At his mother’s Lakeshore Road home, the windows on the front door were covered with pieces of paper.

Before his arrest in January, Renz rented a basement apartment at the Clay Court Apartments complex, according to an employee there. The manager, who retired from Wegmans, said she trusted Renz and thought he was management material. She was astounded at the charges against him.

Read more: Syracus.com