More than two years after a teenage gunman killed 19 children and two teachers at an elementary school in the Texas city of Uvalde, several state and federal investigations have all placed some responsibility for the botched police response on a key figure: the school district’s police chief, Pete Arredondo.

Mr. Arredondo, who was one of the first officers to arrive on scene and acted as the incident commander, failed to mount the “urgent intervention” needed to save more children’s lives, according to a criminal indictment filed against him. Instead of rushing to confront the gunman, various investigations found, he and scores of officers who arrived later waited for shields, backup officers and keys to a classroom door that was ultimately found to be unlocked.

On Monday, Mr. Arredondo is expected to appear in a criminal court for the first time since the tragedy, charged by a grand jury in June with 10 counts of endangering a child for his role in the slow police response. His lawyers are seeking to have the charges dismissed, arguing that he cannot be held criminally liable for actions he did not take, and that the indictment fails to clearly state what actions he did take that put the lives of the victims in danger.

By the time Mr. Arredondo responded to reports of shots fired on May 24, 2022, an active shooter was already “hunting and shooting” the children and the teachers, the lawyers argued in their motion.

Mr. Arredondo has contended that he was not the incident commander, and his lawyers said it does not matter in the context of the indictment. “Such an allegation may invoke a moral duty to perform his job well, but it fails to invoke a legal duty,” the lawyers argued in their filings.

But the indictment said Mr. Arredondo endangered the children by trying to negotiate with the gunman while he “was engaged in an active shooter incident, delaying the response by law enforcement officers to a gunman who was hunting and shooting a child or children.”