Sheriff allegedly guns down judge in his chambers

Investigators are searching for a motive after a Kentucky sheriff was arrested for allegedly gunning down a judge in his chambers, officials said.

Letcher County Sheriff Shawn Stines has been charged with first-degree murder for allegedly shooting and killing Kentucky District Judge Kevin Mullins at the Letcher County Courthouse Thursday afternoon, according to officials.

A 911 caller reported shots being fired inside the courthouse, and responders found Mullins, 54, with multiple gunshot wounds, Kentucky State Police said. He had been shot in his chambers, according to Kentucky Gov. Andy Beshear.

Stines, 43, allegedly shot Mullins "following an argument inside the courthouse," Kentucky State Police said. Officials said they had not determined if Stines used his "duty" weapon.

He was taken into custody at the courthouse without incident.

Stines is cooperating with the authorities conducting the investigation, officials also said. It was unclear if Stines has an attorney.

A motive remains under investigation, Kentucky State Police trooper Matt Gayheart told Johnson City, Tennessee, ABC affiliate WJHL.

No one else was in the judge's chambers at the time of the shooting, Gayheart said.

"It is an isolated incident, and there is no threat to the public," he said.

Kentucky Attorney General Russell Coleman said his office will work with the Commonwealth's Attorney for the 27th Judicial Circuit, Jackie Steele, as special prosecutors in the case.

The courthouse is closed on Friday, according to an order by Letcher County Judge/Executive Terry Adams.

Commonwealth's Attorney for Letcher County Matt Butler recused himself from the case.

Butler not only worked with the sheriff, but had a "close, personal relationship" with the judge, he announced in a self-recorded video on Friday.

Butler and Mullins were formerly brothers-in-law when they were married to a pair of sisters, Butler said.

"His daughters are my nieces. My two oldest kids are his niece and nephew," Butler said. "As a brother-in-law, I will never forget how kind he was to my children."

And when the judge was a prosecutor and Butler was a public defender, they faced off in court, but always stayed friendly outside of court, Butler said

Butler in his message asked people to "pray for everybody involved."

Laurance B. VanMeter, chief justice of the Kentucky Supreme Court, said in a statement he was "shocked by this act of violence," adding the "court system is shaken by this news."

"My prayers are with his family and the Letcher County community as they try to process and mourn this tragic loss. I ask for respect and privacy on their behalf," he said.

"While the investigation is ongoing, we are committed to providing assistance in any way that we can," the Kentucky Court of Justice said in a statement. "Our deepest sympathies go out to all those impacted by this tragic event, and our thoughts and prayers are with the community during this challenging time."

Mullins had served as district court judge for Kentucky's 47th District Court, which presides over Letcher County, since 2009, when he was appointed by then-Gov. Steve Beshear, the father of the current governor.

He was a graduate of the University of Kentucky and earned his law degree from the University of Louisville.

Letcher County, located along the Kentucky-Virginia border, is about 150 miles southeast of Lexington, Kentucky.

ABC News' Victoria Arancio, Jack Date, Darren Reynolds and Leah Sarnoff contributed to this report.