CHEVERLY, Md. — An undercover narcotics officer was mortally wounded by one of his colleagues as he responded to an attack on his police station by a gunman with a death wish, their police chief angrily explained on Monday. And while Officer Jacai Colson lay dying, the gunman’s two brothers coldly recorded Sunday’s firefight on their cellphones, Prince George’s County Police Chief Hank Stawinski said, tapping a podium and trying to contain his emotions. The gunman, Michael Ford, dictated a “last will and testament” just minutes before his two brothers drove him to the station, where he began spraying bullets at passing cars and even an ambulance to draw officers outside, police said. Colson arrived at the station after the firing began.