The families of two black Chicagoans killed by police accused officers on Sunday of having used excessive force and Mayor Rahm Emanuel of having failed them, piling pressure on a city facing a U.S. federal probe over possible racial bias in policing.

Police shot Quintonio LeGrier, 19, a male college student who was visiting his father, and Bettie Jones, a 55-year-old mother of five, on Saturday.

Family members said police were called after LeGrier threatened his father with a metal baseball bat. Jones, who lived in a first-floor apartment, was shot through the door, said her cousin Evelyn Glover.

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Police said LeGrier was being combative. They said Jones was killed by accident and extended condolences.

"This needs to stop. No mother should have to bury her child," Janet Cooksey, LeGrier's mother, told a news conference in front of the home. She said her son was shot seven times. She previously told reporters her son suffered from mental illness.

Several people who spoke wore T-shirts reading, "Rahm Failed Us." Family members demanded to know why police used lethal force.

High-profile killings of black men by police officers since mid-2014 have triggered waves of protest – including in Chicago, the country's third-largest city.

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After the news conference, about 100 people including neighbors and religious leaders held a vigil in neighborhood streets, with many saying they did not trust the police to be truthful about what happened.

"We are under siege here in Chicago," Ira Acree, pastor of the Greater St. John Bible Church, said at the vigil. "Trigger-happy cops are still engaged in senseless murders of people of color."

A Chicago police video of the fatal shooting of another black teenager sparked protests last month, with activists demanding the resignation of Mayor Emanuel, a former chief of staff to President Barack Obama.

A federal investigation is under way over the department's use of deadly force and officer discipline.

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LeGrier, a sophomore at Northern Illinois University, was home for Christmas. His father is the landlord of the two-story wooden frame building where the shooting occurred.

Police said an unspecified weapon was recovered and no officers were hurt. They did not say whether there was a video of the incident, provided no information on officers involved, and said the department was unlikely to say any more on Sunday.