Illinois judge Michael Cook is calling it quits as he battles federal heroin possession charges stemming from the death of a fellow judge who died of a drug overdose at Cook’s cabin.

Cook, a 43-year-old justice for St. Clair County Circuit Court in southwestern Illinois, wrote a brief letter of resignation on Wednesday to Chief Judge John Baricevic, the Associated Press reported. Cook didn't give a reason for quitting in the letter, nor did he mention the heroin charges that he's facing.

The judge is undergoing treatment for drug abuse, according to Reuters.

Baricevic told the AP that Cook’s resignation didn't come as a shock to him. The Illinois judge started out as an associate circuit judge in 2007 and rose to circuit judge in 2010. He was previously an assistant public defender.

"I'm not surprised under the circumstances," he said. "In one sense, it was a formality, but a very important one. Disciplinary procedures seeking him removed could have taken a very long time. "He has accepted responsibility, and that has allowed us to move ahead.”

The judge was charged on Friday with heroin possession and having a gun while illegally using controlled substances. Cook pleaded not guilty to the charges and is free on bond, AP reports.

During his arraignment, the Illinois judge wore cutoff shorts and a T-shirt with “Bad is my middle name” emblazoned on the front, KMOV-TV in St. Louis, Mo., noted.

Charges were filed against Cook after the Pike County coroner ruled on Friday that Cook’s colleague, St. Clair County Judge Joe Christ, died of a cocaine overdose at Cook’s family cabin in March. Christ had only been on the bench for a week before he died.

A federal investigation is ongoing into Christ’s death. A probation officer for St. Clair County admitted to the FBI that he gave cocaine to the Illinois judges right before the hunting trip and that he did so on other occasions as well.

The probation officer, James Fogarty, allegedly admitted to using cocaine with Cook and Christ at the cabin in Pleasant Hill, Ill., near the Mississippi River, and that the three men snorted coke the day before they left for the hunting trip.

Fogarty, who lives in East St. Louis, Mo., was charged with cocaine distribution and drug possession.