THUNDER BAY, Ont. -- Aboriginal leaders say indigenous artist Moses Beaver has died under what they are calling unexplained circumstances.

Beaver, a renowned Woodlands artist, is believed to have died in a jail in Thunder Bay, Ont., Nishnawbe Aski Nation Grand Chief Alvin Fiddler and Nibinamik First Nation Chief Johnny Yellowhead said Thursday in a statement.

Ontario's Ministry of Community Safety and Correctional Services said a male inmate was found unresponsive in his cell at the Thunder Bay jail on Monday night.

Paramedics were called and the inmate was pronounced dead at a hospital, spokesman Andrew Morrison said in an email. Morrison said the matter is under investigation, but gave no other details.

Regional coroner Dr. Michael Wilson said the man who died had been in custody "for a while," but could give no details. Wilson said he had spoken with Yellowhead about the death.

Yellowhead said Beaver had struggled with mental health issues for many years.

"We do not understand why he was in custody or the circumstances that led to his death," Yellowhead said. "It is clear that Moses needed professional help and a psychiatric assessment, and we demand to know why this didn't happen."

Fiddler called the circumstances of the death "troubling."

"We will demand an investigation into the circumstances around his passing," he said.

Beaver's sister, Mary Wabasse, died Wednesday in a collision in Thunder Bay as she was travelling to comfort family members and make funeral arrangements for her brother, Fiddler and Yellowhead added.

"Our community had barely begun to mourn his loss when the news came that his sister Mary was killed in an accident," Yellowhead said.

The Lake Superior Art Gallery in Thunder Bay says on its website that Beaver was a self-taught artist who worked with acrylic on canvas, Indian Ink on paper and watercolour, and calls his use of colour "revealing."

Beaver's work "reflects the black lines of traditional Woodlands art, he embraces his own unique style of embedded images of spirits, human faces and animal forms, transcending physical boundaries to the outer dimensions of the spiritual realm," it said.

The gallery also said he had worked with youth within the educational system and in community projects.