D.C. Mayor Adrian Fenty said “a feeling of horror” came over him after learning that a mentally disabled 65-year-old man languished in cockroach-infested squalor while the city’s bureaucracy missed one opportunity after another to step in.

Mr. Johnson (a pseudonym) died Feb. 23 at Providence Hospital, four days after falling into a diabetic coma. His death came more than two years after a nonprofit social worker first attempted to obtain D.C. government help for him.