Former Boston police officer and union leader Patrick Rose Sr. this week pleaded guilty to sexually abusing six children, but many questions remain about the Boston Police Department and what its members did to cover up Rose's crimes.

Department officials were aware of accusations against Rose in the mid-1990s. Despite a criminal complaint, he remained on the force until his retirement in 2018.

U.S. Attorney Rachael Rollins, appearing on Greater Boston, said she found the case "incredibly troubling."

"It's deeply disturbing to not have answers about how this happened," said Rollins, who is the former district attorney of Suffolk County.

Rose was sentenced to a maximum 13 years in prison.

"That is really devastating, that's less than two years per victim that this individual gets," Rollins told host Jim Braude.

Rollins was also involved in an investigation in Springfield that found patterns of fourth amendment violations by some police officers in the city.

"Excessive violence, strikes to the head, you know, excessive force allegations that were sustained, some of them," Rollins said.

She added that there is now oversight and significant changes are taking place within the department.

Watch: U.S. Attorney Rachael Rollins talks about police misconduct