The doctor who was fatally shot at an El Paso Veterans Affairs clinic on Tuesday had previously filed a threat complaint against his alleged killer, FBI officials said Wednesday.

VA psychologist Timothy Fjordbak, 63, was allegedly shot and killed by Jerry Serrato, 48, on the fourth floor of the El Paso Veteran Affairs Clinic,  FBI special agent Doug Lindquist said. Then Serrato “actually went to the third floor, and that’s where he took his own life,” Lindquist said.

Serrato, who was once in the military, worked at the VA in 2013 as a desk clerk. But the incident that prompted Fjordbak to file a complaint took place in a grocery store, Lindquist said.

“Mr. Serrato approached Dr. Fjordbak, who did not recognize him, and he made a verbal threat,” said Lindquist, who paraphrased the threat as: “‘I know what you did and I will take care of that,’ something to that effect.”

Fjordbak filed the complaint in October 2013, the FBI said.

Lindquist said the complaint was the only connection they have found between the victim and alleged shooter.

The incident at the clinic was reported around 3:10 p.m. local time on Tuesday, said Maj. Gen. Stephen Twitty, commanding general of Fort Bliss. FBI investigators have interviewed 400 witnesses, some of whom were people seeking medical attention.

A former VA employee at the El Paso clinic told The Post in a phone interview that Fjordbak was well regarded at the facility, showing meticulousness and dedication to his work. As the chief psychologist at the clinic, however, he had expressed concern about his own safety even before Serrato had threatened him, the employee said.

Serrato, a former soldier, had expressed anger about being denied a claim of post-traumatic stress, the former VA employee said. VA officials did not find his claim credible, the former employee added. He spoke to The Post on the condition of anonymity out of concern that his comments would result to discipline or retribution for people he knows still working there.

While VA hospitals have some security, the former employee said he and his old colleagues discussed feeling vulnerable at the clinic, which had no metal detectors. Some guards there carry Taser-like electroshock devices, he said, but they can be slow to arrive if someone pushes a panic button to call for help.

The El Paso health-care system has been under investigation for long patient wait times. A recent inspector general review confirmed the problem and said El Paso had the sixth worst wait-time in the vast VA hospital network, which has over 128 in its system.

“They’re overworked, understaffed, underpaid and underequipped,” the former clinic employee said. “Doctors were bringing their pens from home.”

U.S. Army officials said Wednesday that Serrato left the Army as an enlisted specialist, and served as an infantryman. He deployed to Iraq from March to July of 2007 and was medically discharged with undisclosed physical ailments in February 2009.

Serrato initially served in the Ohio Army National Guard from 1985 to 1989 and joined the active-duty Army in July 2006. He served primarily with the 10th Mountain Division’s 89th Cavalry Regiment, 4th Brigade Combat Team at Fort Polk, La.

Serrato did not earn the Purple Heart award for sustaining any injuries in combat, and did not earn the Combat Infantryman Badge, which goes to infantryman who have personally fought in ground combat. His awards include the National Defense Service Medal, the Iraq Campaign Medal with a campaign star, a Global War on Terrorism Service Medal, and the Army Service Ribbon, officials said.

The VA clinic remained closed on Wednesday.

The William Beaumont Army Medical Center, which is adjacent to the El Paso VA main health-care facility, was placed on lockdown Tuesday afternoon after reports of an active shooter situation. Twitty said the VA “immediately enacted their response plan for all persons to shelter in place.” Lindquist described the response to the active shooting as “a model of how to respond.”

In September 2014, Fort Bliss officials said they would be ramping up security measures after a military assessment found the base was out of compliance, the Associated Press reported at the time. The changes included random vehicle checks and limiting access at certain gates.

The VA Clinic is technically a part of the base but not on the main post, a base spokesman said.

The president of the VA employees’ union, David Cox Sr., said in a statement: “Our VA medical centers are where veterans and their families come to heal and feel safe. Incidents such as this violate that sense of protection.”

RELATED: Shooting at VA clinic underscores frustrations between some VA workers and veterans

This post has been updated.