TUCSON, Ariz. — Gunmen stopped a pickup truck full of illegal immigrants, shot several and took the rest captive Thursday in an attack that left at least three men dead and two people wounded, authorities said.

Authorities were trying to determine who the gunmen were.

The men shot three people, one fatally, along a known smuggling corridor near Tucson, then forced the six or seven other immigrants in the group to leave with them, Pima County sheriff's officials said.

The bodies of two of those immigrants were found a few miles north in the cab of the pickup truck that had been carrying the group. The other four or five immigrants had not been found by midday Thursday.

Investigators did not immediately know a motive for the attack, but gangs of bandits are known to roam border areas preying on illegal immigrants as they cross into the country. Feuding among smuggling organizations also is not uncommon, sometimes involving demands for ransoms from the immigrants relatives to gain their freedom.

Thursday morning's attack occurred on a dirt road near the Silverbell Mine, about 20 miles northwest of Tucson, said Rick Kastigar, the Pima County sheriff's criminal investigations chief.

Kastigar said the immigrants were heading north when four men armed with assault rifles in another vehicle forced the truck to stop. During a confrontation, one man was killed, another was shot in the hand — losing several fingers — and a woman was shot in the neck, Kastigar said.

Sheriff's Sgt. James Ogden said the two wounded immigrants were taken to a hospital with "very, very serious injuries."

Two more men who were not injured were taken into custody as witnesses and were questioned by investigators, Ogden said.

Kastigar said the confrontation followed another incident about 12 hours earlier more than 70 miles south, near the border at Sasabe, in which 18 illegal immigrants were robbed at gunpoint by four heavily armed men wearing ski masks.