Twin sisters from Australia were both shot in the head at an outdoor Colorado shooting range, killing one of the women and leaving the other critically injured, authorities said Tuesday.

The Herald Sun, an Australian newspaper, is reporting that the two sisters made a suicide pact.

Authorities are not looking for a suspect, MyFoxColorado.com reports.

Arapahoe County officials have not released the identities of the victims found Monday at the range at Cherry Creek State Park south of Denver. Sheriff's Capt. Louie Perea declined to release the sisters' hometown or other information until their family was notified.

Perea says investigators aren't sure who fired the gun or guns, or whether the same bullet hit both sisters, who were in the same shooting lane. The sisters had taken a taxi to the range and had been there for more than an hour when they were shot.

Perea said it's unclear whether the surviving sister would sufficiently recover to be able to provide information to investigators.

Investigators were working with the U.S. Immigration and Customs Enforcement to determine how long the sisters had been in the United States.

"We're looking at the last several days she was alive, see where they were and we want to confirm some facts," Perea said.

The sisters had a small-caliber weapon at one of the lanes where shooters line up behind a wooden wall and shoot through an opening that resembles a window, Perea said. There was no apparent indication of a dispute between the sisters or anybody entering or leaving the area.

Surveillance cameras captured patrons' reactions to the shooting, but not the shooting itself. A patron notified range officials of the shooting.

"Apparently the two ladies were shot simultaneously," Doug Hamilton, owner of the Family Shooting Center, told MyFoxColorado.com. "The 'how' and the 'why' we really don't know."

Click here for more on this story from MyFoxColorado.com.

The Associated Press contributed to this report.