A Prince George's County Circuit Court judge ruled yesterday that a 22-year-old man who held 12 women hostage in an office at Landover Mall Feb. 9 was not responsible for his actions and should remain institutionalized for further psychiatric evaluation. Under Maryland law, the ruling was the equivalent of a verdict of not guilty by reason of insanity.

Judge Vincent J. Femia found Leonard Thomas Dunmore--whose threat to detonate a briefcase packed with five pipe bombs led to the mall's evacuation and a police rescue of the hostages--guilty of attempting to blow up the office, kidnaping, assault with intent to maim, possession of exposives and the manufacture of explosives.

But the judge ruled that the Glenarden resident could not be held responsible for the acts because he was suffering from severe mental illness at the time they were committed.

Dunmore had pleaded not guilty to all the charges by reasons of insanity.

Femia ordered Dunmore to undergo further psychiatric evaluation to determine whether he continues to be a threat to himself and society and should remain confined. Dunmore has been in Clifton T. Perkins Hospital, the state's maximum security psychiatric hospital, since his arrest.

A state psychiatrist and a clinical psychologist in private practice testified yesterday that Dunmore was suffering from severe mental illness when, dressed in Army fatigues, he entered the office of the Maryland Vocational Rehabilitation Center carrying the briefcase.

The explosives were never detonated and the hostages escaped unharmed after 2 1/2 hours while police negotiated with Dunmore to surrender.

Psychologist Dennis Harrison said Dunmore told him he went to the office because he was angry at the Prince George's County school officials for assigning him to attend a combination hospital-education facility in Texas for the mentally disturbed.

After the verdict, Dunmore, shaking and crying, told the judge, "I was trying to pursue my civil rights, which people tried to prevent me from doing. Even though I faced death, I felt I had to do this." Dunmore said he was reacting to what he felt were abuses from doctors and institutions.