SOUTH PASADENA, Calif. (AP) — Two teenage boys arrested in what police said was a plan for a mass shooting at their Los Angeles-area high school have been charged with new counts of conspiracy to commit murder and conspiracy to commit assault with a deadly weapon, prosecutors said Tuesday.

The charges were filed Monday in Juvenile Court against the 16-year-old and 17-year-old students over the plan involving South Pasadena High School, the Los Angeles County district attorney's office said.

The boys previously were charged with making criminal threats against another boy after allegedly sharing the plan with him. Police and prosecutors said then that more charges were expected.

The names of the suspects have not been released because of their ages. They have denied the threat charges, and a trial has been set for Sept. 25.

Defense attorney Bruce Margolin, who represents one of the boys, said the evidence doesn't support the new charges.

"I think they've gone overboard here," Margolin told KCBS-TV. "No weapon was found or access to any weapon. No attempts to obtain any weapons. . So I think they're going to have problems with this prosecution."

The teens were arrested last month just before the school year began in what the South Pasadena police chief said was a plot to target three staffers and kill as many students as possible.

The teens had no weapons and hadn't set a date for an attack, but police say they had researched automatic firearms, handguns, knives, explosives and tactics.