Demonstrators from the Black Lives Matter movement interrupted the U.S. Conference of Mayors’ meeting on Wednesday, protesting the killing of a Chicago teenager by police and the lead-tainted water supply of Flint, Michigan.

At least three people among onlookers and one person onstage held up signs when Baltimore Mayor Stephanie Rawlings-Blake, the group’s president, spoke during the meeting’s opening news conference at a hotel in Washington, D.C.

One protester interrupted her several times, shouting, "You failed us!" Baltimore was torn in April by rioting over the death of black detainee Freddie Gray, and six police officers are facing charges.

A protester also held up a sign onstage that said, "16 Shots is a cover up. #LaquanMcDonald #ResignRahm."

The 2014 shooting death of Laquan McDonald, a black teenager, by a Chicago police officer has led to calls for Mayor Rahm Emanuel to step down. Emanuel has been the target of protests because of the year-long delay in bringing charges against the officer.

The deaths of Gray and McDonald were among a series of police killings of black men that have fueled the rise of the Black Lives Matter movement and stoked a national debate on race and policing.

At the news conference, Flint Mayor Karen Weaver was interrupted by a protester who shouted, "I need some water!"

Flint is facing a health crisis from lead in its drinking water. The U.S. Environmental Protection Agency is expected to brief staffers of a House of Representatives on Thursday about the contamination.

The protest at the mayors’ meeting went on for about 25 minutes, with occasional outbursts and the display of signs. Protesters left without interference from security.

Rawlings-Blake, Emanuel and Weaver are all Democrats.