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

Lawmakers are responding Monday to an explosive phone call between President Trump and Georgia Secretary of State Brad Raffensperger in which Trump, citing debunked and dismissed conspiracy theories about election fraud in Georgia, repeatedly demanded that Raffensperger "find" enough votes to deliver the president a win in Georgia.

"The people of the country are angry, and there's nothing wrong with saying that, you know, that you've recalculated," Trump said Saturday on the call, first obtained and reported on by The Washington Post but also independently obtained by ABC News. "Fellas, I need 11,000 votes -- give me a break."

Trump went on to vaguely warn of criminal consequences if his claims weren’t pursued, but Raffensperger, rebuffing the president's allegations at the time, repeated to ABC News Chief Anchor George Stephanopoulos on "Good Morning America" Monday that Trump's data is "just plain wrong."

"He had hundreds and hundreds of people he said that were dead that voted. We found two. That's an example of just -- he has bad data," Raffensperger, a Republican, said.

Democrats have widely condemned Trump’s language on the call with Sen. Dick Durbin, D- Ill., arguing the president’s conduct "merits nothing less than a criminal investigation” and Vice President-elect Kamala Harris deeming it a “baldfaced, bold abuse of power by the president of the United States.”

With Trump slated to leave office in 16 days, the repercussions could instead be felt in Georgia -- where a pair of Senate runoffs on Tuesday will determine the balance of power in the U.S. Senate. 

Some Republicans are concerned the president’s repeated claims could suppress turnout in Georgia -- even as he prepares to travel to the state Monday to boost support at an evening "victory rally" in Dalton for sitting Sens. Kelly Loeffler and David Perdue. Biden, meanwhile, is traveling to Atlanta to stump for Democratic contenders Rev. Raphael Warnock and Jon Ossoff. If Democrats win both Senate seats, Harris will cast tie-breaking votes in the upper chamber.

It all comes as at least 12 Senate Republicans are officially preparing to challenge Biden’s Electoral College win in Congress on Wednesday, a stunning development that comes despite the public wishes of Senate Majority Leader Mitch McConnell. Meanwhile, dozens more in the party including Arkansas GOP Sen. Tom Cotton, a Trump loyalist, have deemed the effort either ineffective, dangerous or lacking evidence.

And in an extraordinary rebuke of Trump over the weekend, all 10 living former secretaries of defense in a public [letter] () warned against any move to involve the U.S. military in pursuing claims of election fraud and said “the time for questioning the results” of the election has passed.