Initial reports were that a 'pistol' was used - note the range estimation from where the shooters were positioned. Sounds like a pistol-caliber carbine, possibly scoped.

(CNN)Almost two days after two officers were shot and wounded at a Ferguson, Missouri, protest, investigators were still seeking breaks in the case, authorities said Friday.
The shots rang out from a hill overlooking the station shortly after midnight Wednesday, at the end of a protest against the Ferguson Police Department. Officers saw "muzzle flashes ... about 125 yards away," Belmar said.
http://www.cnn.com/2015/03/13/us/ferguson-protests/