DALLAS—A contract security guard accused of hacking into computers at the clinic where he worked and posting images of it online faces a federal charge, prosecutors said Tuesday.

Jesse William McGraw, of Arlington, was being held on a charge of downloading malicious code into a computer without authorization. He was arrested Friday and was being detained pending a hearing Wednesday, the U.S. attorney’s office said.

McGraw, 25, worked for United Protection Services as an overnight shift security guard at The Carrell Clinic in Dallas, which specializes in orthopedic care.

A message left for The Carrell Clinic was not immediately returned Tuesday. Federal prosecutors say clinic staff identified and fixed the compromised computers.

An affidavit alleges McGraw intruded into clinic systems that controlled climate and ventilation for patients and medications and computers that contained confidential patient information between April and June.

A network security researcher reported the crime. An FBI agent investigating the case states in an affidavit that a review of the climate control computers found a malicious program that allowed remote control of the system by unauthorized users.

Attorney information for McGraw could not immediately be located.

Investigators say McGraw is the leader of the “Electronik Tribulation Army” hacker group and went by the name GhostExodus and used other aliases. McGraw wanted other hackers to help him conduct a massive computer attack before or on July 4 in which they would assume control of some computers and use them to attack others, an affidavit alleges.

Officials also say McGraw bragged about hacking into the systems on online forums and posted video on YouTube of himself infecting computers at a business he claimed to have “infiltrated.” The man in the video had a fake FBI credential, a cell phone jamming device, concealable pen video camera and other “hacker gear,” the affidavit said.