GREENSBURG — A teen charged with slashing and stabbing 20 fellow students and a security guard at a high school is guilty but was mentally ill during the rampage, his attorney said.

But a prosecutor insists the teen is just plain guilty and wasn’t unduly driven by mental illness. So a judge on Monday said he’ll digest testimony from experts and gave attorneys time to file arguments about whether he should accept the teen’s proposed plea to 21 counts each of attempted homicide and aggravated assault.

Both sides agree Alex Hribal walked through Franklin Regional High School in Murrysville, using two kitchen knives. Hribal was 16 when he attacked on April 9, 2014, choosing the birthday of Eric Harris, one of two teens psychiatrists say Hribal “worshipped” for the attacks on Columbine High School.

If the judge accepts that plea, Hribal will be sent to a mental hospital. If the judge rejects the plea, defense attorney Patrick Thomassey said he’ll let a jury decide whether Hribal is guilty but mentally ill, guilty or not guilty by reason of insanity.