some details
Here is a snip from the first news article I found. too bad the Guardian had to label the link in such an inflammatory way, but it is a British newspaper:
http://www.guardian.co.uk/world/2008/apr/26/usa.usgunviolence
"...The events of that November night began when Bell and his friends got into an argument outside the club.
The party had no idea that the Club Kahlua was under police surveillance as part of an investigation into prostitution. Oliver, Isnora and Cooper had recently come on shift when the argument erupted. A number of witnesses testified that they heard talk of guns during the argument, and that Guzman had said: "Yo, go get my gun."
The detectives decided to follow Bell and his friends out to their car. They told the court they feared that talk of guns during the argument could lead to a shooting. According to Isnora, he then identified himself as a police detective, clipping his badge on his collar and shouting out: "Police".
But Guzman and Benefield said they heard no warning and had no idea that Isnora was with the police when he walked up to their car with his gun drawn. They say the gunfire erupted without any warning.
The police, for their part, claimed that Guzman may have helped to provoke the shooting by urging Bell to drive away. The police claim that they opened fire only after Bell nudged Isnora with his car, pushing him into an unmarked police van.
Oliver then fired at Bell's car 31 times. Isnora fired 11 shots and Cooper four."
***It seems that since the police were undercover, they weren't in uniform. Put yourself in the place of the people who got shot: an argument happens with some guys outside a bar. You decide to just get away. One of your group says they have a gun possibly to deter the presumed other thugs. I can understand not realizing the guys were cops, and can also believe they didn't hear or believe it when one office supposedly ID'd himself as police. So they tried to drive away, clipped one of the cops, then all hell broke loose.