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

House Speaker Nancy Pelosi, in a new letter to Democrats on Saturday, 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 on Monday.

Pelosi thanked fellow Democratic representatives for speaking with her on a conference call Friday and said she was taking their considerations into account. She did not specifically mention voting on impeachment in the letter, though.

"Since that call, I have received scores of communications from Members expressing your views about how we go forward and regarding a strong presence of our Caucus in the Capitol," she wrote. "Over the last few days, I have discussed your views with Constitutional lawyers, both inside and outside the Congress, to consider the parliamentary and constitutional options available to us."

"From what I have heard from Members, and from the deluge that I have received from the public, it is clear that, once again, the Times Have Found Us to save our democracy," Pelosi added in the letter. "We will be proceeding with meetings with Members and Constitutional experts and others. I continue to welcome your comments. I urge you to be prepared to return to Washington this week."

While Democrats are hopeful Republicans will back their effort, none have come out and said they would back the charge as drafted.

Pelosi said at a press conference on Thursday, "If the Vice President and Cabinet do not act [on the 25th Amendment], the Congress may be prepared to move forward with impeachment."

Some Democrats planned to connect with Republican colleagues this weekend to gauge their support and encourage them to back the effort.

-ABC News' Benjamin Siegel