Green’s Dictionary of Slang

(US) a detective, in later 20C esp. a member of the drug squad.

1929		E. Dahlberg Bottom Dogs 73: Two days later a d.t. brought back Mush Tate; a bull had caught him on a M.K.T. freight headin’ for Chi.
1986		C. Stroud Close Pursuit (1988) 98: You be in crime two year and you don’ know no DT when you be seein’ him.
1988		Slick Rick ‘Children’s Story’ 🎵 Tried to rob a man who was a DT undercvover.
1995		S. Moore In The Cut 122: D.T.’s, detectives.
2006		(con. 1972) Jurgenson & Cea Circle of Six xvii: Randy Jurgensen, the omnipresent spoke on a wheel of detectives or DTs.