A man who had been on Nebraska’s death row since 2003 died Saturday, reducing the total number of condemned inmates in the state to 11, prison officials said.

Arthur Gales, 55, was sentenced to death for the November 2000 slayings of his then-girlfriend’s two children. Authorities said he killed 13-year-old Latara Chandler and 7-year-old Tramar Chandler because they were potential witnesses to him severely beating their mother.

The Nebraska Department of Correctional Services said in a news release Monday that the cause of Gales’ death wasn’t yet known, but he had been undergoing treatment for an unspecified medical condition at the Tecumseh State Correctional Institution, where the state’s death row is located. A grand jury will investigate, which happens anytime a prison inmate dies in custody.

Gales also received a 50-year sentence for the attempted murder of Judith Chandler, whom authorities said was left for dead outside her Omaha apartment.

Autopsies revealed that Latara died of strangulation and Tramar died of drowning and strangulation. Authorities said Latara had been sexually assaulted, and a pathologist testified at trial that each child had been strangled for at least four minutes.

Gales had not been scheduled for an execution, nor have any of the 11 remaining men on Nebraska’s death row. The state’s last execution was in August 2018,  and prison officials have acknowledged that they don’t have any more lethal injection drugs and aren’t likely to get any.

Before 2018, Nebraska’s last execution took place in 1997. In 2015, death row inmate Michael Ryan died from complications related to cancer.

Nebraska could add two more inmates to death row this year with the sentencing of Aubrey Trail and Bailey Boswell, who were convicted in the 2017 slaying of a Lincoln woman who disappeared after a Tinder date.