Special Agent Mike Martinez declined to identify the doctor or the suspected gunman, who was also shot dead in the incident Tuesday afternoon at the El Paso Veterans Affairs Health Care System clinic on Fort Bliss.
Agents on Wednesday began the mammoth task of questioning the hundreds of patients, staff and others who were present at the clinic when the shooting took place. Investigators have revealed few details of the attack, including a possible motive or whether the gunman killed himself or was killed by someone else.
The FBI, which is leading the investigation because the shooting occurred on federal property, will release more information at a news conference Wednesday afternoon at the agency's El Paso office, Martinez said.
"We're trying to expeditiously get through those hundreds of witnesses to find out details about this incident," said Douglas Lindquist, FBI special agent in charge of the El Paso office.
The shooting comes just four months after the Fort Bliss Commanding Officer Maj. Gen. Stephen Twitty announced new security measures after a military assessment found the base was not fully in compliance with Department of Defense directives. The measures included random vehicle checks and limiting access to Defense Department personnel at some gates. However, four gates still remained open to the public, according to a press release.
On Wednesday, civilians were still able to access the post with only a driver's license, passing through just a single checkpoint manned by several soldiers. Investigators talked outside the closed VA clinic, warning of broken glass, while soldiers entered the adjacent William Beaumont Army Medical Center.
Sutton Smith, a worker at the VA clinic, said that a "code white" was issued over the intercom system indicating an active shooter and ordering people to seek shelter.
Smith said he hid with about a dozen people in a locked room with the lights off for some two hours. Apart from the initial alert and some communication among managers via cellphone, no official updates were provided during the lockdown, he said.
The El Paso clinic came under scrutiny last year after a federal audit showed it had among the longest wait times for veterans trying to see a doctor for the first time. A survey last year of more than 690 veterans living in El Paso County found that they waited an average of more than two months to see a Veterans Affairs mental health professional and even longer to see a physician.
It was not clear if that scandal was linked to the shootings.
The VA said in a statement that it was "deeply saddened" by the attack and was assisting in investigations.
"The safety and continued care of our veterans and the staff will be our focus throughout," the agency said.
Robbins reported from Dallas.