Texas church shooting victims and their families could have a hard time successfully suing the government over the Air Force’s failure to submit the gunman’s criminal history to the FBI — a step that would have blocked him from legally buying weapons.

Relatives of the more than two dozen churchgoers killed in Sunday’s attack would face major obstacles, including protections written into law to shield the military from certain legal action.

An easier route might be to seek help from Congress, which could pass a law acknowledging the Air Force’s mistake and offering compensation.

A look at some key issues:

Who to sue

There are just two plausible targets for a lawsuit, legal experts say: Devin Patrick Kelley’s estate and the Air Force.

At issue would be Kelley’s purchase of the Ruger AR-15 used in the attack. The Air Force did not report his 2012 court-martial for abusing his wife and her child to a federal database used for background checks on gun purchasers. Had that conviction been entered into the database, as required by Pentagon rules, Kelley’s purchase of any gun would have been denied.

Cause and effect

The question at the core of any civil case would be: Did the clerical failure by the Air Force cause the deaths in Texas?

Proving such a direct link wouldn’t be easy, said Antonio M. Romanucci, a leading personal-injury attorney and principal and partner with Romanucci & Blandin LLC.

“In a regular negligence case, you only have to prove some act was ‘a’ cause of an injury,” he explained. But according to federal statutes that lay out the ground rules for suing the government, “you have to show it is ‘the’ cause of the injury. … That is a much, much higher standard.”

Chances of success

If a lawsuit is filed, the government could choose to seek a settlement before the case gets to trial. But federal authorities have typically dug in to fight litigation for fear that giving in too easily would invite a flood of similar litigation.

One risk of going to trial would be that sympathetic jurors end up awarding plaintiffs far more in damages than they might have been willing to settle for earlier.

“You can’t think of a more sympathetic group of individuals — children and worshippers on a Sunday gunned down,” Romanucci said. “What they will get … you are looking at [a potential award of] tens and ten of millions of dollars.”