Story highlights A border patrol spokesman says it's probable they are immigrants

The bodies have not been identified, the Maricopa County Sheriff's Office says

Activists say border deaths are on the rise as security forces more dangerous crossings

Border patrol officials have argued that more security is necessary to stop smugglers

Authorities found four human bodies abandoned in the Arizona desert Thursday.

Initial indications point to exposure as the probable cause of death, said Victor Brabble, a spokesman for U.S. Customs and Border Protection.

"It is probable that they are immigrants attempting to cross into the U.S.," he said. "However, we don't have enough to draw a conclusion on it now."

The bodies were found near Gila Bend, Arizona, about 70 miles north of the border. They have not been identified, the Maricopa County Sheriff's Office said.

Immigrants often try to cross the Sonoran Desert's harsh terrain, and there are many heat-related deaths, Brabble said Thursday.

Immigrant rights advocates have warned that even as border crossings decrease, deaths are on the rise as increased border security forces people to choose more dangerous crossing routes. Border patrol officials have argued that more security is necessary to stop smugglers.

According to a recent study by the Binational Migration Institute at the University of Arizona, more than 2,230 migrants have died in the state's desert area along the border in the past 22 years.

In the border region of Pima County, Arizona, deaths of unidentified migrants in the desert have become so common the Medical Examiner's Office has helped create a website to track the deaths and assist family members searching for their loved ones' remains

On Thursday, a bipartisan group of senators announced a proposal to add 20,000 more border agents, complete 700 miles of fence along the boundary with Mexico, and deploy $3.2 billion in technology upgrades similar to equipment used by U.S. forces in Iraq and Afghanistan.

The proposed amendment, negotiated by a group of senators from both parties known as the "Gang of Eight," is intended to ensure Senate passage of a major immigration reform bill with enough Republican support to persuade the GOP-controlled House to also take up the measure.