The mother of the man whose remains have been found in several locations in Minneapolis in recent weeks said Wednesday a piece of her son's skull has been sent to the FBI for examination.

Also Wednesday, the Hennepin County Medical Examiner's Office confirmed that the remains found in all four places belong to 36-year-old Adam R. Johnson.

The mystery about what led to Johnson's death and dismemberment began June 17, when a passerby called 911 to report the discovery of remains behind the Ukrainian American Community Center in the 300 block of NE. Main Street, not far from the Mississippi River. Later that same day, police recovered more remains nearby at NE. 3rd and University avenues.

On June 22, a passerby called 911 after spotting the remains about 6:45 a.m. on or near a bench at E. Franklin Avenue and West River Parkway along the river.

The most recent find occurred on July 4 in the river across from downtown and near the old Pillsbury mill in the St. Anthony Main area of the city.

Police have yet to make an arrest in the case, police spokesman John Elder said Wednesday.

Lisa Sanchez, who lives in Utah, said the Medical Examiner's Office "did send into the FBI a skull specimen" from her son that was recovered in hopes of learning more about Johnson's death.

FBI spokesman Kevin Smith declined to comment about the case.

Otherwise, Sanchez said, "it's been a waiting game that is frustrating right now."

Sanchez said that police "have some suspicions and evidence that look promising," but she's not expecting any significant developments anytime soon.

Police are treating Johnson's death as a homicide, but otherwise they've said nothing about where he was killed or the circumstances.

Johnson did not have a permanent address since moving out of a relative's home in Inver Grove Heights in June.

Sanchez said she is raising reward money through GoFundMe.

Police continue to urge anyone with information about the case to call CrimeStoppers at 1-800-222-TIPS (8477).

Sanchez said she last connected on June 13 with her son, whose had a few scrapes with the law over the years. That was four days before the first of his remains were found.

"I just asked him, 'Are you tired of jail yet?' "

She said he replied, "'Yeah, but better [times] to come.'"

Paul Walsh • 612-673-4482