Yale's might have been one of the first assassinations via Tommygun, but they had been used in Chicago prior to Yale's demise. Since Thompson submachine guns fired the same .45 ACP rounds as the common Colt Model 1911 pistol, police investigators collecting evidence couldn't always tell which weapon was used unless they had witnesses. And Thompsons were relatively new and unfamiliar to most people at the time. They were designed to be "trench brooms" in WWI, but the armistice was declared before any were shipped overseas. The US War Department and law enforcement agencies weren't interested in them, so relatively few of them were in circulation.
You can find something about Lovett and Yale at this site:
http://members.fortunecity.com/moran9/id35.htm
The writing is piss-poor, but he does have a few facts (mostly gleaned from Bergreen's Capone biography).