The series of meetings come as Clinton finalizes her running mate selection process, a source familiar with the process told CNN, and hours after presumptive Republican nominee Donald Trump officially named Indiana Gov. Mike Pence as his No. 2.
Campaign chairman John Podesta, who is heading Clinton's VP vetting process, and campaign vice chair Huma Abedin were also in the house during the meetings.
Brian Fallon, a Clinton campaign spokesman, would only say in a statement that Clinton "held a series of campaign-related meetings at her Washington home, including several about her vice presidential selection process."
Cars were seen going in and out of the house on Whitehaven Street near Dupont Circle, with occupants entering through a protected garage. Warren's was the fourth car to arrive at the home, and she visited for roughly an hour.
A liberal stalwart known for championing issues related to economic inequality, Warren is on Clinton's short list of possible vice presidential picks, according to Democrats familiar with the process. The Clinton campaign is aware that Warren's addition to the ticket could galvanize the left wing of the Democratic Party, particularly supporters of Vermont Sen. Bernie Sanders, her primary competitor.
But Warren has been seen as an unlikely pick to Democrats inside and outside Clinton's campaign, in part because the two don't know each other well and there are concerns that Warren -- a darling with the left -- could overshadow the top of the ticket.
Clinton and Warren campaigned together for the first time in 2016 in Cincinnati last month. The two, according to aides, also met over coffee before that campaign stop.
Castro, Hickenlooper also visit
A source familiar with the vice presidential vetting process told CNN Castro also visited Friday, but didn't provide further details. A message left with Castro's office was not immediately returned. His meeting with Clinton was first reported
by U.S. News and World Report.
Castro, an early front-runner for the job, is seen as young, vibrant and someone would further cement the Latino vote. But his experience is far less than anyone else on Clinton's list and some Democrats fear he could be cast as a lightweight.
And Trump's poor polling among Latinos suggest the need to consolidate Latinos is no longer a top concern for the campaign.
As for Hickenlooper, who met with Clinton for more than an hour, aides say the former secretary of state personally likes the Colorado governor and he is seen as someone who could help deliver Colorado, a crucial battleground state.
Asked after the meeting how his time with Clinton was, the governor declined to comment.
Clinton held an event on tech investment last month in Denver and met with the popular Colorado governor over breakfast before the event, according to aides.
Clinton poring over VP research
Clinton spent a full day last week poring through reams of research about potential running mates before she began a final series of interviews and appearances with her top vice presidential choices. Cheryl Mills, a longtime confidant who is taking a lead role in the vetting process, and James Hamilton, a veteran Washington lawyer who is overseeing the teams of lawyers researching the candidates, joined Clinton last Thursday as she met with teams of lawyers researching the candidates.
Clinton is leaning toward announcing her decision immediately after the Republican convention ends on July 21. But aides say she is leaving open the possibility of waiting until Democrats are already gathering for their convention in Philadelphia the following week to disclose her selection, hoping to stoke drama and build interest in the Democratic ticket.
Clinton has been coy about her possible VP pick, pointing to the preparedness for the job, diversity and competence as top priorities.
"I want to be sure that whoever I pick could be president immediately if something were to happen," Clinton told CNN last month. "That's the most important qualification."