Attorney General Eric Holder on Sunday ordered a federal medical examiner to perform another autopsy on a black Missouri teenager whose fatal shooting by a white police officer has spurred a week of rancorous and sometimes-violent protests in suburban St. Louis.

The ‘‘extraordinary circumstances’’ surrounding the death of 18-year-old Michael Brown and a request by Brown’s family members prompted the order, Department of Justice spokesman Brian Fallon said in a statement.

‘‘This independent examination will take place as soon as possible,’’ Fallon said. ‘‘Even after it is complete, Justice Department officials still plan to take the state-performed autopsy into account in the course of their investigation.’’

The Justice Department already had deepened its civil rights investigation into the shooting. Officials said a day earlier that 40 FBI agents were going door-to-door gathering information in the Ferguson, Missouri, neighborhood where Brown, who was unarmed, was shot to death Aug. 9 in the street.