Gunmen stopped a pickup truck full of illegal immigrants, shot some 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 and said some of the immigrants remained missing.

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

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The bodies of two of those immigrants were found about 10 miles north in the cab of the pickup truck that had been carrying the group.

Two men who were not injured were in custody and telling deputes what happened, and the other immigrants had not been found by midday Thursday.

The crime was similar to a robbery Wednesday night more than 70 miles south, near the Mexican border, said Rick Kastigar, the Pima County sheriff's criminal investigations chief. In that incident, four men carrying assault weapons and wearing ski masks robbed 18 immigrants, but no one was reported injured.

Investigators did not immediately know a motive for Thursday's 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.

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

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