A Russian-speaking immigrant who police said may have been practicing dentistry without a license for years in Oregon has been charged with murder in the shooting death of a man believed to be a patient.

Viktor Grigorevich Gebauer, 79, has pleaded not guilty in the death of another Russian-speaking immigrant, Viktor Merezhnikov, 47, whose body was found on a chair in what a prosecutor described as a makeshift waiting room at Gebauer's home in suburban Gresham.

Chris Ramras, a Multnomah County deputy district attorney, said no motive for the Feb. 18 shooting has been determined.

"We may never know," Ramras said Wednesday.

He said both men had emigrated from the former Soviet Union, but he did not know where, or whether they were ethnic Russians.

Robert Axford, the court-appointed attorney for Gebauer, declined to comment on Wednesday.

In court documents, Ramras said the victim called his brother, Vladimir Merezhnikov, to say he was being held at gunpoint by Gebauer, who was threatening to kill him.

Vladimir Merezhnikov did not know where Gebauer lived, and he contacted Viktor Merezhnikov's daughter, Olga, and his wife, Raisa, and they went to the Gebauer home.

They told police that Gebauer opened the door holding a gun and Vladimir Merezhnikov could see his brother slumped in a chair.

Vladimir Merezhnikov said Gebauer threatened to shoot him before closing the door, and the family of the victim called police.

After a Russian-speaking officer arrived, Gebauer emerged and surrendered. The victim was found shot in the abdomen, according to court documents.

Gebauer told police he had never seen Viktor Merezhnikov before the shooting, and claimed the victim tried to rob him using a knife.

But the victim's wife, Raisa, told police that she and her husband knew Gebauer and had been to his home before. She also said her husband did not own a knife like the one found in his left hand, and that he was right-handed.

Also found in Gebauer's home were a dental chair and dental equipment.

The Oregon Board of Dentistry had been investigating reports of an unlicensed dentist practicing at the Gebauer address in 2009 who was believed to be the same man who was reported practicing in 2000 in the Salem area.

After the shooting, Gresham police told the board that Gebauer had used other last names similar to the names of the man in the 2000 and 2009 reports.

Patrick Braatz, Board of Dentistry executive director, said Wednesday that unlicensed dentistry is rare in Oregon, with only three cases investigated during his seven years with the board.

Braatz noted that unlicensed dentistry is a misdemeanor, and the board can only report its findings to police and be pursued as a criminal case.

Sgt. Rick Wilson, Gresham police spokesman, said a detective investigated the 2009 report but found little cooperation from the Russian immigrant community.

"He was working it in between felony cases and said he just wasn't getting anywhere," Wilson said.