A 17-year-old accused of plotting a massacre at his Massachusetts school told police he had wanted the event to be ‘‘bigger than Columbine’’ High School’s 1999 tragedy in Colorado.

A 17-year-old American girl who allegedly agreed to smuggle guns into her school for a rampage, but later leaked the plan to a teacher, has become the fourth teen charged in the case.

A 16-year-old boy is expected to be charged today in connection with what US officials claim was a plan to kill teachers and students at a high school in Massachusetts.