TUCSON, Ariz. —(AP)— Ranchers who rounded up and held 27 illegal immigrants from Mexico for authorities are under investigation, the U.S. attorney's office acknowledged.

Roger Barnett and his brothers captured the group on their Douglas-area ranch on April 4. Armed with handguns, the ranchers held the group until the migrants could be turned over to the Border Patrol. None of those held complained of mistreatment, and they were later returned to Mexico.

On Tuesday, however, protesters who gathered outside the U.S. attorney's office here sought an investigation.

``We believe there may be a conspiracy here to violate civil rights,'' said Isabel Garcia, a board member of the Coalicion de Derechos Humanos, also known as Arizona Border Rights Project.

Garcia said Barnett and dozens of other Cochise County residents signed a statement in March warning that bloodshed could result if illegal immigration were not controlled.

Nearby, counter-demonstrators said the ranchers were justified in taking action to protect their property and themselves.

Cathy Colbert, a spokeswoman for the U.S. attorney in Phoenix, said there is an investigation underway but declined to characterize it as a civil rights probe.

Barnett said authorities have a right to investigate, but added, ``My rights were violated, too. I get 'em violated every day.''

``It seems like the government's protecting everyone else's rights but the American citizen's,'' he said.

Douglas-area residents say illegal entrants damage fences and water lines and leave a trail of litter that sometimes harms livestock.

``When you see 30 and 40 people coming at you out in the middle of your pasture, it's pretty frightening,'' one said.

In January, a Santa Cruz County rancher in a wheelchair is believed to have wounded an illegal immigrant as a group of 30 people crossed his property. He faces charges stemming from the incident.