Sunday morning hikers were despondent to see saguaros and boulders in Saguaro National Park East had been vandalized over the weekend.
The letters "SOMA" were sprayed on several locations along the Desert Spring Trail.
"People love the park, and most of the hikers this time of year are locals and they were very upset," said Joe Sharkey, a volunteer at the park who hands out trail maps. "What kind of a malicious person would do that?"
He said hikers coming down the trail stopped to tell him about the graffiti, and he hiked up to see it for himself.
"That's shocking," he said. "That's not the kind of thing that you should see on the trail."
Sharkey reported the incident to national park rangers.
A person who answered the telephone at the ranger station Sunday said he could not comment. Messages left for the chief ranger were not returned.
Saguaro National Park has a "Park Watch" program - similar to a neighborhood watch - and anyone with information on vandalism is asked to call the ranger station at 733-5113.
Vandalizing a national park is a federal offense punishable by up to a $5,000 fine, six months in prison and three years of probation.