The late-night theft of a Border Patrol truck, a tripped sensor and an unforeseen encounter with another agent led a Nogales agent to plead guilty to smuggling marijuana.

Alex Peña, 37, pleaded guilty Feb. 22 to agreeing to help smuggle marijuana brought across the U.S.-Mexico border in exchange for money. He faces up to two years in prison and is scheduled to be sentenced May 16, according to documents filed in U.S. District Court in Tucson.

While in uniform, Peña stole a Border Patrol truck early in the morning of Aug. 2, 2016, and drove it to a remote area south of Patagonia, where authorities said he intended to help smuggle marijuana. Authorities did not seize any marijuana, but Peña admitted to conspiring to smuggle more than 130 pounds of marijuana, according to the plea agreement.

Peña joined the Border Patrol in November 2008 and resigned March 27, 2017, the same day he was placed on unpaid, indefinite suspension and five days after he was indicted by a federal grand jury, according to a statement from the agency’s Tucson Sector.

His bank records showed Peña regularly deposited large amounts of cash starting in 2013, including $30,000 deposited in 2016 that was not from his salary, according to an affidavit filed by a special agent with the Department of Homeland Security’s Office of the Inspector General.

The affidavit, filed with a February 2017 request for a search warrant of Pena’s phone, described an elaborate plan to meet drug smugglers near the border and deliver marijuana to Tucson.

The plan started to unravel around 2:30 a.m. when a sensor was tripped in hills about eight miles north of the border patrolled by Sonoita station Border Patrol agents.

After hearing about the tripped sensor on his radio, a Sonoita agent went to investigate the sensor and saw the headlights of another Border Patrol truck, which pulled up next to him near the intersection of Forest Road 58 and Hale Ranch Road.

Peña told the Sonoita agent he was from the Nogales station and he was looking for a place to sleep for a few hours before he and other Nogales agents installed a surveillance device on Red Mountain. This was routine practice so the agent was not alarmed, according to the affidavit.

Peña then apologized for tripping the sensor. The comment made the Sonoita agent suspicious because it indicated Peña was listening to the Sonoita station’s radio transmissions.

After Peña drove away, the Sonoita agent followed tire tracks in the mud for a quarter-mile before finding an area of trampled grass where it appeared a truck had backed up.

Agents found footprints of four to six people, greasy hamburger wrappers, soda cans, and a camouflage canteen, all of which were indicators of “load-out” sites where drug smugglers wait to transfer loads to drivers, according to the affidavit. The footprints led agents south toward Mexico.

Around 3:30 a.m., the truck driven by Peña went through the Border Patrol checkpoint on Interstate 19. Video footage showed the driver briefly turned on the truck’s emergency lights and agents waved the truck through the checkpoint.

Peña called and sent text messages to a Tucson phone number throughout the night. Phone records showed he had been contacting this number for about a year. The number was used in 2013 to contact the target of a Drug Enforcement Administration investigation, according to the affidavit.

The owner of the phone Peña called, who was not charged as part of the conspiracy, bought a house a few miles from the load-out site 11 days before Peña picked up drugs at the site.

Investigators used cellphone towers to track Peña’s route on Aug. 2, 2016. His phone’s signal was tracked to locations and times corresponding to Peña leaving his house in Tucson, taking the truck from the Nogales station, and speaking with the Sonoita agent near Patagonia.

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The phone signal also was picked up by a tower near the I-19 checkpoint around the time agents waved the truck through the checkpoint. The signal then went to Tucson and returned to Nogales.

After the Sonoita agent reported finding the load-out site, agents found the truck Peña drove in a side parking lot at the Nogales station, locked, with the hood still warm. The keys to the truck were missing and have not been found.

The truck was found with mud covering its tires, running boards, and the floor mats. The last agent who checked out the truck reported he returned it clean. Drug-sniffing dogs found no drugs in the truck.

Video footage showed an agent parking and entering the station. The driver used an access card assigned to Peña. Both the Sonoita agent and Peña’s supervisor at the Nogales station said the man in the video and in a photograph spread likely was Peña.

Peña wore a uniform and drove a marked truck to meet the smugglers that night, which is unusual because he normally worked the day shift in plain clothes and drove an unmarked vehicle for a specialized unit, according to the affidavit. He showed up for his normal shift less than an hour after returning the truck.

Peña has been released on pretrial supervision since March 2017, court records show.

Agents perform their duties with honor and distinction and “any arrest of an employee undermines that hard work and dedication,” Tucson Sector Chief Patrol Agent Rodolfo Karisch said in a prepared statement.

Peña’s defense lawyer Chris Scileppi would not comment.

Contact reporter Curt Prendergast at 573-4224 or cprendergast@tucson.com or on Twitter @CurtTucsonStar