SAN FRANCISCO -- The district attorney dropped all charges against police chief Earl Sanders and his top assistant Tuesday, saying he couldn't prove they helped orchestrate a cover-up of a street brawl involving off-duty officers.

District Attorney Terence Hallinan said he was dropping felony counts of conspiracy to obstruct justice against Sanders and assistant chief Alex Fagan Sr. "in the interest of justice" after reviewing 1,300 pages of evidence.

"I felt there was probable cause" to support the charges, Hallinan said. But he said he couldn't prove guilt "with an abiding certainty."

Hallinan said he would still prosecute others charged in the case that has rocked the department and the city. Five police commanders still face conspiracy charges, and three rookie officers are charged with felony assault for allegedly attacking two men Nov. 20 over their bag of steak fajitas.

All the officers have pleaded innocent. One of those charged with assault is Alex Fagan Jr., the assistant chief's son.