HARTFORD — The state ­Supreme Court rejected Monday the ­appeal of a mentally ill murder defendant who was ­ordered into state custody in 2010 after officials discovered that he had been released from a state mental hospital 18 years earlier without their knowledge.

Justices upheld lower court rulings in the case of Pedro ­Custodio, 79, who was found incompetent to stand trial in the 1991 shooting death of his Waterbury neighbor Amerigo Pagan Cruz. Custodio was sent to a state mental hospital in 1992, but was released only a few months later after hospital officials concluded he was no longer a danger to himself or others.