Eric Garner, 43, whose videotaped confrontation with police has caused widespread outcry and calls by the Rev. Al Sharpton for federal prosecution, was killed by "the compression of his chest and prone positioning during physical restraint by police," said medical examiner spokeswoman Julie Bolcer.Asthma, heart disease and obesity were contributing factors, she said. Chokeholds are prohibited by the New York Police Department. The case is being investigated by prosecutors on Staten Island, though Attorney General Eric Holder has said the Justice Department is "closely monitoring" the investigation. The NYPD didn't immediately respond to a request for comment on the medical examiner's ruling. The officer who put Garner in the chokehold was stripped of his gun and badge pending the investigation, and another was placed on desk duty. Two paramedics and two EMTs were suspended without pay. Police Commissioner William Bratton has said the officer appeared to have placed Garner in a chokehold and has ordered a top-to-bottom redesigning of use-of-force training in the NYPD. In provocative comments Thursday, Sharpton called for the officers to be charged criminally. Sharpton believes chokeholds are used disproportionately on minorities.