AIR FORCE ACADEMY, Colo. — Investigators were unable to determine who left a cross at an outdoor worship space for pagans at the Air Force Academy, school officials said Friday.

The investigation is closed and no arrests were made, academy spokeswoman Capt. Corinna Jones said.

The large wooden cross was left in January at a double circle of stones on the academy grounds near Colorado Springs, about two weeks before the academy announced the site will be set aside for pagans and other Earth-centered religions.

Investigators found no clear indication the cross was intended to harass the groups because it was found before the announcement of the site's new use, and because the site has sometimes been used by Christians and by other groups, Jones said.

Tech. Sgt. Brandon Longcrier, who sponsors the Earth-centered group, said he's satisfied the investigation was thorough.

"It felt like something that was done maliciously, but without the evidence, I can't say either way if it was a hate crime," Longcrier said. "We'll just hope it doesn't happen again."

Jones said investigators don't consider the incident a hate crime because they were unable to determine a motive, and because the Earth-centered group decided not to file charges.

"For a hate crime, you have to know a motive. You don't know a motive if you don't know who did it," she said.

Jones said investigators talked with Longcrier, seven other members of the group and the academy chaplain staff.

Religious tolerance became a sensitive issue at the academy when a 2004 survey found that many cadets were hearing slurs or jokes about other religions and that some felt ostracized because they weren't religious.

Commanders say they have made tolerance a priority and that the climate has improved.