You are probably thinking of Fred "Killer" Burke and James Ray, or "Screwy" Claude Maddox.

Burke was a member of STL Egan's Rats, a predominantly Irish gang that also included Jews and Italians. Witnesses placed Burke near the St Valentines scene and guns seized from his home in 1929 were matched to bullets from the crime.

James Ray was an friend of Burkes and was also placed at the scene by witnesses. Burke and Ray had dressed as cops before when committing crimes.

Maddox was also a member of STL Egan's Rats, and had moved to Chicago. At the time of the St Valentines Day Massacre, he owned the Circus Cafe and was aligned with Capone. One of cars used by the killers was traced back to him.

IMO, Burke was one of the St Valentines Day Massacre shooters. There is just too much evidence. I am not sure who else was there, but Ray probably was, or was a lookout.

As a side note, Maddox's place became the name for the crew (the Circus Cafe Gang) that included Anthony "Tough Tony" Capezio, Vincenzo De Mora ("Machine Gun" Jack McGurn) and Antonino "Tony" "Joe Batters" Accardo.