Police are investigating the shooting deaths of two American citizens, gunned down while waiting to come into the U.S. at a border crossing near San Diego.

According to the Baja, Calif., attorney general’s office, the two men were waiting in a white pickup truck at the San Ysidro border crossing on the Tijuana side when an unidentified man walked through the lanes of traffic and boldly unloaded five rounds from a 9-mm. handgun. 

The men were shot in the head, chest and arms. The shooting happened on Monday at 2:40 a.m. Customs and Border protection agents say it appears the shooting was too far away from surveillance cameras, which point southbound into Mexico, for investigators to use for clues.

The victims were identified as 25-year-old Sergio Salcido Luna and 28-year-old Kevin Joel Romero. It’s believed the pair, who may have dual citizenship, were on their way to work at the West Coast Beverage Maintenance, a company that services draft beer equipment for bars and restaurants.

Both men were described by those who knew them as “hardworking, good guys” who worked as much overtime as possible and lived on the Mexican side of the border to save money.

Luna was a mixed martial arts fighter and friends say he was training for a fight in June.

"He was a cage fighter who went by the name Sergio 'Suave' Salcido. He's a really tough fighter, old school type fighter, old school brawler, hard nose, real tough guy," fellow fighter Evan Anthony told KSWB.

Law enforcement tells Fox News neither man had a criminal record, although Mexican police say a small amount of drugs were found on one victim.

While shootings like this are rare, the Bureau of Alcohol, Tobacco, Firearms and Explosives and U.S. Immigration and Customs Enforcement agents who spoke with Fox News on the condition of anonymity doubt this was a random shooting or a case of mistaken identity. 

Agents say it’s possible these men were moving loads of drugs and didn’t pay their debts, ripped off one of the cartels or tried to cheat someone on a drug deal. Another possibility is that the cartels believed these men to be confidential informants for American law enforcement.

In the past, Americans were seen as off limits by the cartels, but not anymore. Along with the recent killings of Border Patrol agent Brian Terry and ICE special agent Jaime Zapata, cartels have also targeted U.S. citizens who they believe have cheated them. In October, a Chandler, Ariz., man was beheaded after stealing 400 pounds of marijuana.