WASHINGTON (AP) — The Justice Department has begun an internal review to determine how to remove any extremists from within federal law enforcement following the arrest of current and former police officers for their involvement in the Jan. 6 attack on the Capitol, Attorney General Merrick Garland said Wednesday.

Garland, in response to a question during a Senate hearing on domestic extremism, described a review that was in its early stages and is complicated by the need to avoid violating the First Amendment rights of Justice Department employees.

The deputy attorney general, Lisa Monaco, “has met with the heads of all of our law enforcement agencies to determine how we can carefully vet our own employees,” he told the Senate Appropriations Committee.

It was a notable disclosure considering that the Justice Department is charged with enforcing federal civil rights laws and oversees the FBI, which is the lead agency in charge of investigating the growing threat posed by violent domestic extremists.

It is also potentially tricky legal ground because of the risk of intrusion on personal beliefs that are protected under the Constitution.

Garland described those competing interests as "being mindful of First Amendment free associational rights, but at the same time being careful that we don’t have people in our ranks who commit criminal acts or who are not able to carry out their duties.”

The Department of Homeland Security last month announced a similar review aimed at determining the extent of any presence of violent extremists within its ranks. Secretary Alejandro Mayorkas, who also testified at Wednesday's hearing on the broader efforts by their agencies to address the growing threat from extremism, told the committee that the results of that analysis would be publicly released.

The attorney general's disclosure of an internal review came in response to a question from Illinois Sen. Dick Durbin about the arrest of a retired New York Police Department officer, Thomas Webster, in the Jan. 6 attack on the Capitol in which supporters of President Donald Trump sought to force Congress to overturn the results of the November election.

Webster, who was captured on video tackling a police officer and striking him with a metal flagpole, was charged with six counts.

Durbin said Webster's arrest raises a “painful” question: whether there are others in state, local or federal law enforcement who might be capable of extremist behavior.

Garland suggested that federal grants could be issued to local and state police departments to help them vet potential officers.

Also during the hearing, he described the broader Capitol insurrection investigation, with more than 400 arrests to date, as far from complete as authorities comb through video and other evidence.

“This investigation is not over,” he said. “We will pursue each lead until we’re confident that we will have reached the end.”

At times during the hearing, Republicans sought to shift discussion to the increasing number of migrants apprehended at the southwest border or press Garland on whether the Justice Department was improperly making the Capitol investigation a priority over other matters, including violence during last summer's social justice protests.

The attorney general called the Jan. 6 attack an attempt to interfere with the peaceful transfer of power, a “fundamental element of our democracy," and thus worthy of attention.

“I have not seen a more dangerous threat to democracy than the invasion of the Capitol," he said.

At least 10 of the people charged for involvement in the insurrection were current or former law enforcement officers at the time of their alleged offenses.

An Associated Press survey of law enforcement agencies nationwide found that at least 31 officers in 12 states are being scrutinized by their supervisors for their behavior in Washington either in the riot itself or the march and protest that preceded it.

Officials are looking into whether the officers violated any laws or policies or participated in the violence.