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

Georgia election official Gabriel Sterling, a Republican, at a press conference on what he called "anti-disinformation Monday," ran through major voter conspiracies pushed by Trump, his allies and far-right media outlets and debunked them one-by-one in an effort to restore faith in Georgia's election system.

"The reason I'm having to stand here today is because there are people in positions of authority and respect who have said their vote didn't count and it's not true," Sterling said, stressing to Georgians that their votes count ahead of critical runoffs in the state which will determine the balance of power in the U.S. Senate.

"It's Whac-A-Mole again. It is Groundhog Day again. I'm going to talk about the things I've talked about repeatedly for two months, but I'm going to do it for one last time," Sterling said, adding he "screamed" at the radio upon hearing audio of the phone call between Trump and Secretary of State Brad Raffensperger because theories Trump questioned have already been "thoroughly debunked."

For example, Sterling, responding to those who claim there were roughly 2,500 people who voted without being registered, said, "Let's just be clear about this: you can't do it!" He said, "So that number is zero," going on to debunk other theories.

Asked if he believes, as some have said, that Trump's phone call was an attack on democracy, Sterling said he'd leave others to make that decision before adding he felt it was "out of place."

"I found it to be something that was not normal -- out of place -- and nobody I know who would be president would do something like that to a secretary of state," Sterling said.

Asked about Raffenperger's desire to have the phone call recorded and whether he was concerned about anything improper being said or needing to release it later, Sterling said it was recorded "out of an abundance of caution"

"I think given the environment we're in right now, the political situation that we're in, the history of the president, knowing that he sometimes doesn't necessarily characterize things as they might have actually occurred, it was out of abundance of caution," Sterling said.

"I'm sure the president's side may have recorded it, too. They may have been the ones who leaked part of that, too," he added.