Three days after Capitol Police Officer Brian Sicknick died defending the Capitol against insurrectionists the White House lowered the U.S. flag to half-staff.

The decision to lower the flags came hours after Capitol Police also announced the off-duty death of Officer Howard Liebengood. The circumstances of his death have not been made public.

The U.S. attorney's office in Washington opened a federal murder investigation into the death of Sicknick, who died Thursday night after suffering injuries in the violent siege on Capitol Hill, three law enforcement sources confirmed to ABC News.

The investigation is being conducted jointly between the FBI and the D.C. Metropolitan Police Department, with cooperation from U.S. Capitol Police.
Acting Attorney General Jeffrey Rosen said in a statement that the Department of Justice "will spare no resources in investigating and holding accountable those responsible."

-ABC News' Elizabeth Thomas

Rep. David Cicilline, D-R.I., said on Twitter that an article of impeachment has just passed 200 cosponsors.

House Democrats were circulating the draft, citing "incitement of insurrection" on Friday and said they could introduce it as early as Monday and hold a vote as early as the middle of the week.

On Saturday, House Speaker Nancy Pelosi, in a letter to Democrats, asked members to prepare to return to Washington this week -- another signal that the House could take up and pass the impeachment article to the Senate after it is formally introduced.

The article charges Trump with "willfully inciting violence against the government of the United States" with his comments at the rally outside the White House that it says "encouraged—and foreseeably resulted in—imminent lawless action at the Capitol."

-ABC News' Benjamin Siegel

While Many Democrats have renewed calls to impeach Trump, following his supporters' storming of the U.S. Capitol on Wednesday, Illinois Republican Rep. Adam Kinzinger said on ABC's "This Week" that he doesn't think impeachment is "the smart move" right now.

He did, however, call for the president to resign or be removed by the 25th Amendment.

"I think it victimizes Donald Trump again and I think there's a moment that we're in right now where Donald Trump, he's looking really, really bad," Kinzinger told ABC News Chief Anchor George Stephanopoulos.

"I'll vote the right way, you know, if I'm presented with that, I just think it's probably not the smartest move right now but I think that's going to be out of my hands," he added.

Rep. Alexandria Ocasio-Cortez, D-N.Y. said she believes that a second impeachment of President Donald Trump should be scheduled.

"Our main priority is to ensure the removal of Donald Trump as president of the United States," Ocasio-Cortez told ABC News Chief Anchor George Stephanopoulos on Sunday. "Every minute and every hour that he is in office represents a clear and present danger, not just to the United States Congress but frankly to the country."

"We're also talking about complete barring of the president -- or rather of Donald Trump -- from running for office ever again," she added. "And in addition to that the potential ability to prevent pardoning himself from those charges that he was impeached for."

Trump 'represents a clear and present danger' to Congress, country: Ocasio-Cortez

At District of Columbia Mayor Muriel Bowser's request, Army Secretary Ryan McCarthy has activated about 340 unarmed D.C. National Guardsmen to assist local law enforcement with the demonstrations taking place on Wednesday pegged to the Electoral College certification.

"We think it's helpful to have our D.C. Guardsmen, who are our Guard, that I have requested from the Secretary of the Army to assist MPD [Metropolitan Police Department] with traffic management," Bowser said Monday. "It absolutely frees up more officers."

The D.C. Guard is unique in that any call-ups have to be made by the Army secretary and not the jurisdiction’s top elected official -- as is the case with state governors. Additionally, more focus has been given to the call-ups since the controversial presence of D.C. Guardsmen and the use of D.C. Guard helicopters during the Black Lives Matter demonstrations last summer.

"They will be deployed to assist us with crowd management, as well as traffic control in our nation's capital that will allow for the police officers to focus on anyone who's intent on instigating agitating or participating in violence in our city," said D.C. Police Chief Robert Contee. He also labeled any possibility of violence surrounding the protests as "unacceptable" and repeated that carrying firearms is not allowed under D.C. law.

A senior defense official said Bowser put in the request for Guardsmen on Dec. 31 and that it was approved Monday morning. Beginning Tuesday through Thursday, the 340 personnel will help man 30 traffic control checkpoints around the northern side of the downtown area close to the National Mall.

"No one's going to be armed or body armor or anything like that," said the official. "What they need is traffic control."

-ABC News' Luis Martinez and Quinn Owen