Democrats were left furious after the US Air Force admitted it had not reported a criminal conviction for the Texas gunman.
At least 26 people were killed after a shooter opened fire on a church in Sutherland Springs outside of San Antonio.
The suspect - identified as Devin Patrick Kelley - was later found in his car a few miles away, dead from a gunshot wound.
As details began to emerge about the 26-year-old, it was discovered he had been dishonourably discharged from the US Air Force after he had been court-martialed for assaulting his wife and child.
Once the service had admitted not entering this into federal databases, angry Democrats began calling on the Trump administration to investigate.
Senator Kirsten Gillibrand - who wrote to James Mattis to request proceedings into it - said Kelley might have been blocked from committing the massacre if the conviction had been properly reported, which would have limited his ability to buy a gun.
Joaquin Castro, who represents the 20th district of Texas, said the failure to report the court-martial was a "deadly" and "colossal" failure.