Abstract: A lock mechanism for releasably locking a security bar (14) over an opening (12) through a wall (10) includes a housing (30, 32, 60) for mounting to the wall adjacent the opening. An engagement mechanism (28) is coupled to the security bar (14). A keeper member (26) in the housing is movable to selectively engage a lock surface (102) on the engagement mechanism to restrain the engagement mechanism from movement relative to the housing; the engagement mechanism being allowed to move unrestrained relative to the housing when the keeper member does not engage the lock surface. A manually-operable latch mechanism (24) in the housing moves the keeper member out of engagement with the lock surface.