Officials said Sergio Luna, a 25-year-old mixed-martial-arts fighter known in the ring as Sergio "Suave" Salcido, and Kevin Romero, 28, were shot and killed as they sat in their pick-up truck waiting to cross into San Ysidro from Tijuana.
Both men lived in Tijuana to save money and worked at West Coast Beverage maintenance in San Diego.
"They were best friends and they both lived in Tijuana," said Marcela Romero, Luna's aunt. "They both came to work every morning like clockwork. At 1 a.m., they were at the border."
Their boss, Matt Pelot, said they worked for him for more than a year servicing draft beer equipment and he was not aware of any problems.
Romero was adopting his girlfriend's 5-year-old son.
Salcido was preparing for a fight in June.
Mexican authorities said the victims may have been targeted by the person who walked up between waiting vehicles and shot them repeatedly with a handgun. The motive remains unclear.
Mexican authorities said a small amount of drugs was found on one of the men, but ruled out drug trafficking as a possible motive.
Officials said nearly 35,000 people have been murdered inside Mexico in drug-related killings in the last four years.
In the first six months of 2010, the latest State Department figures available, 49 Americans were victims of homicide in Mexico, up from 37 for the same period in 2009 and 19 in the first half of 2008.
However, shootings while waiting to cross the border are rare.
"It scares me because I cross maybe four times a week by myself," said Carolina Fonseca of Tijuana.
Romero's aunt claims it may have been revenge stemming from a lovers triangle involving Salcido, his girlfriend and another man.
"In December, Kevin, Sergio and the other guy got into a fight because she was caught with him and he threatened him, 'I'm going to get you. I'm going to kill you,'" the aunt said.
The Associated Press contributed to this report.