(CNN) Since an ex-US airman shot more than two dozen people in a Texas church in November, the US military has added more than 4,000 names to the nation's list of dishonorably discharged military personnel banned from owning firearms -- a sign of what has been a massive hole in the nation's gun buying background check system.

The gunman in the Sutherland Springs massacre had been kicked out of the military for assaulting his wife. By federal law, that should have prevented the shooter from purchasing his semiautomatic rifle, but the US Air Force later admitted it had not submitted his records to the FBI's background check system.

In the months since, the US Department of Defense has scrambled to ensure all of its branches have properly updated the FBI's system to track personnel kicked out of the military who are barred from owning firearms.

That push, a CNN review has found, has uncovered a backlog so significant that the FBI's tally of dishonorably discharged former service members has ballooned by 4,284 names in just three months, a 38% leap.

The FBI figures track the reasons civilians and ex-military personnel are barred from owning guns. The agency separately quantifies dishonorable discharges, which includes personnel convicted by a general court-martial. Other types of military dismissals that could legally stop someone from owning a gun are not broken out from the civilian population in the FBI data.

Since 2015, the number of people barred from owning firearms because they were dishonorably discharged had hovered at about 11,000, according to FBI statistics published online. That number suddenly jumped to 14,825 last November , then to 15,583 in December . It now stands at 15,597.

The Defense Department has not yet publicly acknowledged that the military has vastly expanded its submissions since the shooting. The belatedly filed reports mean that, for an unknown period, more than 4,000 people had the opportunity to buy guns from dealers while they should have been legally barred from it.

"I'm encouraged that they're trying to hurry up and get through this backlog. But it was a failure of duty and responsibility to not report these people to the federal database. I'm highly disappointed," said US Rep. Scott Taylor (R-Virginia), a former Navy SEAL now working on a bill to improve the background check system.

New scrutiny across military branches

Dishonorable discharges are reserved for people convicted by the military of violence or serious misconduct — crimes equivalent to a felony — and thus federally prohibited from owning guns. The armed forces are also required to report personnel who receive a "bad-conduct discharge" for causes such as domestic violence or dealing drugs, which also disqualifies them from gun ownership. The Texas church shooter received a bad-conduct discharge . But only dishonorable discharges are reflected in the recent jump.

CNN tracked the sharp increase in dishonorable discharges in the FBI's records by reviewing archived pages of the agency's website, which publishes occasional updates of the data within its background check system.

In the wake of the Texas shooting, the Pentagon's Office of Inspector General is reviewing how the branches of the military submit information about dishonorable discharges to the FBI.

CNN reached out to all five branches of the military. The US Marine Corps, Air Force and Navy acknowledged that they have been combing through old records.

"We are in the process of conducting a thorough review of past cases to ensure that any prior failures to report are rectified and the appropriate information is provided to the FBI," said Capt. Christopher R. Harrison, a spokesman for the Marines. He said the Marine Corps was planning changes that would "increase the speed and efficacy of reporting."

The Air Force has been under particular scrutiny because of the Texas case. Secretary of the Air Force Heather Wilson told Congress in December that her military branch is reviewing cases dating back to 2002.

The US Navy pointed to a December DOD Inspector General's report, unrelated to the Texas shooting, in which it promised to conduct a thorough search of cases dating back to 1998 to make sure all missing files are sent to the FBI.

The US Army said it would be "inappropriate" to comment, citing a pending lawsuit about its failures to report these types of cases.

The US Coast Guard assured CNN its numbers weren't included in this recent spike, because it always reports to the FBI on time. And no Coast Guard personnel were dishonorably discharged in November or December, it said.

"We can't speak to what the other services are doing. The Coast Guard puts information into the FBI database upon receiving the results of trial. We are not clearing a backlog of personnel," said spokeswoman Alana Miller.

The Pentagon's Office of Inspector General is currently reviewing how the Air Force failed to process the case file for Texas shooter Devin Patrick Kelley. It is also reviewing how the branches of the military submit relevant information to the FBI.

The church in Sutherland Springs, Texas, where a man killed 25 people and an unborn child on November 5. Within weeks, the armed forces began adding thousands of names to the FBI's gun ban list of people who, like the shooter, had been kicked out of the military.

Even before its release, however, the 4,000-person backlog hints at a problem the military has long known about — and has not completely fixed.

1997 inspector general report noted that the military was often neglecting to notify the FBI when someone was convicted. The Navy failed to do so 93% of the time; the Army, 79% of the time. The report's authors blamed poor policies that put "little emphasis" on sharing the information. But failure to report dishonorable discharges continued to be common, highlighted by similar IG reports in 2015 and 2017

The 2017 IG report showed that the military branches are failing to report the final status of 31% of cases to the FBI. Firearms data experts said the sudden addition of 4,000 names might mean the reporting has now caught up.

Officials at the section of the FBI that runs the National Instant Criminal Background Check System, called NICS, declined to say why the figures have spiked so quickly.

"The NICS Section does not speculate on changes in statistics," said FBI spokesman Stephen G. Fischer.

Extensive flaws in gun background checks

The nation's firearm blacklist is limited in myriad ways. For one thing, it extends only to guns purchased at licensed dealers. When someone who should not be able to buy a gun tries to purchase one there, FBI examiners have only a nerve-wracking three days in which to complete a background check, drawing on fragmented and incomplete federal and local records. Some states extend that time period, but it's still a race against the clock. Federal agencies are required by law to turn over prohibitory records to the FBI, but as the military's failures show, they sometimes do not.

Licensed dealers, of course, are not the only route to a firearm. In many states, people can buy a gun from a neighbor, acquaintance or local trader without having to undergo a background check.

Still, an estimated 78% of gun owners undergo background checks , so blocking dishonorably discharged personnel from buying guns through dealers is important.

"The background check system has blocked over three million illegal attempts to buy guns. But a database is only as good as the data in it," said Avery W. Gardiner, co-president of the Brady Campaign to Prevent Gun Violence, which advocates for stronger government regulation of firearms.

As much as the sudden additions to the federal database point to a backlog, Gardiner pointed out, it's also a sign the armed forces are trying to get the record up to date.

"It seems the military worked quickly after the Sutherland Springs massacre to get its data into the background check system. I hope other state and federal agencies aren't waiting for the next high-profile shooting before making sure that their data is in the background check system too," Gardiner said.

Two difficult questions now: Did these 4,000-plus people kicked out of the military buy a gun before they were added to the ban list? And if they bought guns, is there is any way to retrieve them?

There's no national registry of firearm owners, due to gun owners' fears about being tracked by government. And by law, when a person passes the FBI background check, the FBI destroys that file. It means it's difficult, if not impossible, to track legal purchases by owner — unless a state, like California or Connecticut, has a registry of all gun sales.

The Bureau of Alcohol, Tobacco, Firearms and Explosives is typically the agency that recovers guns that fall into the wrong hands, but it has not been ordered to check whether this group purchased firearms. The law enforcement agency relies on referrals from the FBI, but the FBI only sends over names of people who failed the background check. Until last November, any of the newly added people might have lied on a dealer's federal form and passed the background check without a problem.

"There is no mechanism to conduct retroactive background checks on approved transactions. ATF investigates when it receives reliable, verifiable information that a person prohibited by federal law possesses a firearm," said Frank Kelsey, an ATF spokesman.

Rep. Scott Taylor (R-VA) is working on a bill to improve the background check system. "It was a failure of duty and responsibility to not report these people to the federal database," he said of the military's dishonorable discharge backlog.

Congressman Taylor said he's worried about those who "slipped through the cracks."

"I'm a rigorous defender of the Second Amendment," Taylor said. "But it is troubling. If you're not supposed to have a gun, you're not supposed to have a gun."

Taylor has co-sponsored the Domestic Violence Loophole Closure Act , which would give the DOD a three-day deadline to report any service members convicted of domestic violence to the FBI's database.

The extent of the military's failures could soon be laid bare. The cities of New York, Philadelphia and San Francisco have joined forces to sue the Defense Department and others in federal court, claiming the military's carelessness has put its citizens at risk. The lawsuit seeks to subject the military to civilian court oversight until the underreporting problem is fixed. In court documents, the government calls the attempt "drastic" and "inappropriate," saying the cities are trying to "intercede in ongoing agency compliance efforts." It plans to file a motion to dismiss. Last month, the judge ordered the Pentagon to search for any internal records that explain the department's ongoing failure.

"There's very good reason to believe those databases remain dangerously incomplete," said Ken Taber, the lead lawyer representing those cities.

Taber, for one, doubts the federal government will find a way to recover any guns from the thousands whose dishonorable discharges went unnoted for years. That would require figuring out whether each of those people purchased firearms, and then tracking them down — something he says is unlikely to ever happen.

"I think there's a huge public policy concern," he said. "I'd like to know if they'll get them. But I'm not holding my breath."