The mayor of Uvalde, Texas, is calling on District Attorney Christina Mitchell to resign and filed a lawsuit accusing her of repeatedly blocking the city's investigation into last year's shooting at Robb Elementary School that left 19 students and two teachers dead.

"It's been fifteen months since this tragedy, and I feel the families and our community deserve answers," Uvalde Mayor Don McLaughlin said in a written statement attached to a lawsuit filed Tuesday. Mitchell "has been involved in a cover-up regarding the City's investigation into the Robb School tragedy," McLaughlin alleged.

Mitchell is leading the criminal investigation into the shooting and law enforcement response, telling the San Antonio Express-News on Tuesday that she is still poring over evidence but intends to present her case to a grand jury by the end of the year.

CNN has reached out to Mitchell for comment on the lawsuit. CNN affiliate KSAT reported she did not respond to its request for comment Wednesday.

In June 2022, Mitchell told CNN she did not want records or videos released while investigations were ongoing. "Any release of records to that incident at this time would interfere with said ongoing investigation and would impede a thorough and complete investigation," she said in a statement.

A total of 376 law enforcement officers from multiple agencies responded to the May 2022 shooting, but waited 77 minutes before storming in and killing the gunman, an 18-year-old Uvalde resident, who was holed up in two adjoining classrooms with the victims. During that time, terrified students begged 911 operators for help as police remained in a hallway.

In the aftermath, residents, state leaders and lawmakers criticized the police response as well as a lack of transparency regarding internal investigations.

This week's lawsuit marks the second time McLaughlin has sued the district attorney, accusing her of refusing to share evidence with the city's independent investigator, Jesse Prado.

The first suit was filed in December and was voluntarily dropped after Mitchell promised to cooperate with Prado's requests for evidence, McLaughlin said.

"The City had to file suit a second time, because D.A. Christina Mitchell continues to block the City's investigation," the mayor's statement said.

The US Department of Justice is also reviewing the law enforcement response to the deadly rampage.