A shootout at a border wall construction site near San Diego that wounded two Mexican security guards last summer is now under FBI investigation, according to private contractors and government documents obtained by The Washington Post.

The incident, which has not been publicly disclosed, occurred on the night of July 1 east of the San Ysidro border crossing, when Mexican security guards came under fire while protecting materials and equipment for Texas-based Ultimate Concrete, according to a U.S. Army Corps of Engineers report.

One of the guards was shot in the lower right buttock, and another suffered a light shoulder wound. The assailants were not identified, but security camera footage of the incident showed that a group of six or seven gunmen approached a job site where U.S. contractors had been installing steel barriers during the day.

"Three men in the vicinity of the border wall immediately opened fire" on the two security contractors, according to the Army Corps account. "Both men took immediate cover and began returning fire," repelling the assailants.

For U.S. authorities, the episode raised questions about use-of-force rules for the Mexican companies hired to protect southern access to worksites where U.S. crews are building Trump's wall. The report also describes concerns raised by U.S. Border Patrol agents who encountered Mexican security guards crossing back and forth across the international boundary without authorization.

SLS, the primary contractor that hired Ultimate Concrete, revised its security protocols after the incident to make sure its security personnel on both sides of the border have met U.S. screening standards, the report stated.

FBI agents made an unannounced visit to the San Diego area offices of SLS on Jan. 22, and executives from the company immediately sent a letter to the Army Corps expressing shock and concern that federal investigators had arrived to ask questions about the shooting.

SLS has been awarded contracts worth more than $1.5 billion for barrier construction at multiple locations, government records show.

Liz Rogers, a spokeswoman for SLS, said the company is cooperating with investigators. She declined to provide details because the investigation is ongoing.

"SLS fully complied with the FBI's requests and voluntarily answered all questions," said Rogers, who is the marketing director for the firm, which is based in Galveston, Texas. "The agents were very professional and the entirety of their visit was less than one hour."

The FBI also issued a subpoena to Jesse Guzman, who owns Ultimate Concrete, documents show. Reached by telephone, Guzman referred questions to the FBI.

Officials with the Army Corps referred questions to the FBI. The FBI and U.S. Customs and Border Protection did not respond to requests for comment on the incident or the investigation.

According to the Army Corps report, dated July 29, Ultimate Concrete hired the Pinkerton agency to provide security at the job site, but the two men injured in the shooting were working for another Mexico-based firm, NSSP.

The shooting victims received medical treatment and had "returned to full duty," the report states.

NSSP had not been vetted by the Army Corps or U.S. Customs and Border Protection to work at the job site, according to the report, a step considered unnecessary because the firm was hired to work on the south side of the border.

"When NSSP personnel were interviewed, they stated they were all prior Mexican National Army and had been trained on when to use deadly force," the report states.

The Mexican security guards would take snapshots of the worksite every evening when they came on duty to guard equipment and materials overnight, largely to show supervisors the following morning that nothing had been missing.

NSSP personnel communicated with a security manager for Ultimate Concrete via the WhatsApp messaging app but sometimes strayed north across the border onto the job site, according to the report.

U.S. border agents saw the Mexican security agents crossing into and out of the United States, making what appeared to be unauthorized entries into U.S. territory. At a job site that essentially straddled an open border, the crisscrossing Mexican security guards drew the attention of U.S. agents.

"The concern was that there were two Mexican nationals 'either posing as or working for Pinkerton on the north side of the border wall,' and that both the Ultimate Concrete and Pinkerton contractors on site denied any knowledge of additional security being hired," according to the report. "When confronted, the Mexican national security personnel again reiterated they were Pinkerton employees."

The report found that the Mexican security guards did not violate rules of engagement during the shootout, having come under attack from assailants before returning fire.

Construction progress on Trump's new "border wall system" is more advanced in the San Diego area than nearly anywhere else the government has added new fencing, resulting in the biggest and most elaborate barrier system along the entire 2,000-mile boundary with Mexico.

Across most of the San Diego border area, a "primary" steel bollard fence is backed by a "secondary" 30-foot barrier, with a road running between them that allows U.S. agents to corral smugglers and migrants in a dedicated "enforcement zone."

The Tijuana-San Diego area is the busiest and most lucrative drug trafficking corridor on the border, according to U.S. officials, and mafia violence on the Mexican side has surged in recent years. Border Patrol agents and other CBP personnel working in the area did not make changes to their security posture after the shooting incident, according to the report.

The Washington Post's Matt Zapotosky contributed to this report.