PHOENIX (Reuters) - Unknown gunmen fired on a truck packed with suspected illegal immigrants in southern Arizona early on Friday, killing two people and wounding a third, police said.

The gunmen killed a man and a woman traveling in a pickup truck near Green Valley, some 30 miles south of Tucson, Pima County Sheriff's Department said.

Spokeswoman Dawn Barkman said the dead were believed to be Mexican immigrants traveling with three family groups from the poor southern Mexican state of Chiapas.

Barkman said two children were among at least 14 people packed into the extended-cab pickup truck.

Each year U.S. border police catch more than a million undocumented immigrants crossing north from Mexico, many of them through the desert corridor south of Tucson. There are an estimated 12 million illegal immigrants in the United States.

Last month gunmen shot dead three Guatemalan immigrants and wounded two others in an attack on a back road northwest of Tucson. Investigators in that incident said rival people-trafficking networks were likely responsible.

Police say increased security on the U.S.-Mexico border has made human trafficking a more profitable activity, resulting in increased violence among smugglers.

A smuggling ring that Arizona police say flew thousands of illegal immigrants across the United States from the Phoenix area was busted earlier this week with 16 people indicted, authorities said on Thursday.

Authorities said nearly $2 million in bookings were made by the ring.
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