Okay, that took a lot more reading than I thought it would.
It would appear that the first police to use semi-auto pistols were in California around 1966; they adopted the Colt 1911 .45. However, the Illinois State Police are credited with setting the trend in 1967 when they adopted a 9mm. But the big change seems to have been in the 1980's, when most police departments mandated a switch from revolvers to semi-autos.
Most of the documents I found seem to credit a rise in violent crime with police demanding weapons with more ammunition capacity and stopping power. One report in "The Encyclopedia of Police Science" said that police in the 1880's typically carried billy clubs; but when working alone, were being jumped by multiple criminals, and therefore resorted to carrying firearms (i.e., escalation of criminal activity brought about requirement for better criminal activity determent).
The same source identified several studies that tried to link gun homicides with availability of weapons; one study of 1986 Detroit showed that the homicide rate would have been 4% lower with 1980 gun ownership levels (less access to guns); but at the same time, the study indicated Detroit's 1986 homicide rate would still be 45% higher. A similar 1979 study indicated that a 10% decrease in a city population's gun ownership would reduce the rate of robbery murder by only 4%. The conclusion of the multiple studies was that limiting access to guns may not cut murder rates as much as gun control advocates hope for.
I sorted the Bureau of Justice statistics for 2004 by percentage of gun-related homicides. To my surprise, Louisiana took the #1 spot, with 79.5% of their homicides being committed by guns (465 murders). Illinois, which I've previously used in this thread as an example because of their gun ownership restrictions since the 1960's, came in #2 with 75.2% (583 murders). Massachusetts, which reportedly has some of the strictest gun control laws in the country, reported 58.1% (98 murders). The lowest rate was Vermont, with 18.8% (3 murders).
North Dakota is credited as being one of the states with the least amount of gun control; and the same statistics from 2004 show that there were 9 murders in the state, with 50% attributed to guns. (4.5 people?)
Anyway, there are lots more statistics out there; and I assume there are some that are more recent than what I've been able to find so far.