Story highlights Jeffrey Williams, 20, is charged with two counts of first-degree assault, prosecutor says

Prosecutor thanks public for leading police to .40-caliber allegedly used in shooting

Officers shot after protests in Ferguson; neither officer from Ferguson Police Department

(CNN) A 20-year-old man from the St. Louis area has been arrested in connection with the shooting of two police officers during last week's protests in Ferguson, Missouri, a prosecutor said Sunday.

Jeffrey Williams was arrested late Saturday, and he has been charged with two counts of first-degree assault, a count of firing a weapon from a vehicle and three counts of armed criminal activity, St. Louis County Prosecuting Attorney Bob McCulloch said at a news conference in Clayton.

"The demonstrations were pretty much over (when the officers were shot). People were leaving, and that's when this happened," he said, describing Williams as a demonstrator who had taken part in protests on numerous occasions.

At the time of his arrest, Williams was on probation for receiving stolen property, and McCulloch said he believed Williams had an outstanding warrant after not reporting to his probation officer for several months.

The prosecutor repeatedly thanked the public for the information that led to the arrest. He also said that, because of the public's assistance in the case, police were able to serve a search warrant on Williams' residence where they seized a .40-caliber handgun, "which has been tied to the shell casings that were recovered" at the scene of the shooting.

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