You can always start up a "Knock 'n' talk" conversation: "Excuse me sir..."
He's not going to stick around, or if he does and he just killed somone, it's going to be to blow your head off. Assuming he doesn't and just drives away, you still don't have any reason to stop him. Besides, you think the guy just killed someone and you're going to walk right up to the van? Tell me honestly that if a run came out like that, you and just about every experienced cop you know wouldn't handle it like a felony stop from the beginning....Honestly, with a straight face. How many cops do you know who would disregard their adrenaline and say "Hmmmmm...we don't know who called this in because he was so scared he ran. That means we don't have reasonable suspicion, absent corroboration. We better not treat this like a felonly stop because we don't technically have reasonable suspicion. I'll stay behind the engine of our car, and YOU go up there and talk to them, but don't make them think that they're not free to leave!"
Personally, I MIGHT wait for a pretextual traffic stop, but then you just let him out of the reasonable security of the parking lot and now he's on the road and you risk a running gun battle and there's a lot better chance that bullets aren't going to aimed as well, increasing the chances shot bystanders. Because if he's shooting at me, I'm shooting back...Moving cars or no moving cars...AND there's 3 of them and only one of you if you let your partner out, two if you stuck together.
Frankly, stopping without even ID'ing them while your partner checked inside would be pretty reasonable, and I think that it would fly with most courts. It's a least intrusive method.
I don't think it's reasonable to split up when there are three guys who are presumably armed in the van, and you have no authority to make them stick around waiting for back-up even if it were available.
I'm curious about what kind of call takes priority over a possible capital murder and robbery in progress?
A couple things here. They're getting in the van NOW, and you think they're armed. How long are you going to wait for back-up while they formulate a plan on how use their AK-47's to take the two cops out who are following them. You won't even see it coming once they get into the van, especially if the windows are blacked out. (Check Shawn Bandy, Detroit PD)
http://www.detnews.com/2002/specialreport/0212/13/a11-30113.htm
This isn't posted to draw on any similarities other than the fact that there were armed guys, in a van, who were willing to shoot the police. I understand that they had probable cause to stop this van, and maybe could have done things differently, but that's not the point. The point in this example is that you can have the same kind of guys in a van, ready to kill you and not have reasonable suspicion enough to do what you need to do to stay alive based on a call from an anyonymous caller.
Also, you can't say for sure the call is for real considering how many BS calls for a felony in progress come out of the ghetto. They're not going to treat it the same way as they would in a town with more resources. True stories: I chased a car with my partner with a bunch of 15 year-olds in it. We chased for 10 minutes. No back-up. They crashed and bailed out. I chased after the kid with the AR-15 and my partner went after one of the other 3 who had a shotgun. I chased my guy through a neighborhood with no street lights. Called for back-up, none available and I didn't hear any sirens in the background. Called for a helicopter, none available. Called for the dogs, wouldn't authorize the overtime. The ONLY back-up I had for 20 minutes while I was looking for a guy with an assault rifle in a dark neighborhood was an off-duty cop from a neighboring city who just so happened to be in the neighborhood and saw me chasing.
Another one: Got a call to an armed robbery in progress of two old ladies in front of the church at 7:45 am. Shift change was at 7:45. We called it out when we spotted the guy, my partner chased him through the yards, and I circled the block and we caught him in about 10 minutes. Everyone else was already cleaning out their cars for off-duty. They figured it was another half-hour old run and that we'd never catch anyone so no one bothered to come out to help us.
Another one: Off-duty cop from another cith is robbed and shot at in a park. He comes 20 feet out of that city to use the payphone to call the police. He gets the 911 operator who tells them he has to call the other city (the pay phone caller ID shows him in city A when the crime took place 20 feet away in city B. He tries to explain, but the 911 operator is to freaking stupid to understand, and insists he has to call city A. City A police go into City B, look for the bad guy, all the while calling city B police to come help. City B shows up an hour later, after the bad guy has been in custody for a half hour!
In short, yes, around here, you can have a shooting run and not have any back-up for all intents and purposes until it's too late, or not needed anymore.
Not to mention the possibility prior to doing their armed robbery, the bad guys called in an "officer in trouble" for a location on the other side of the precinct. Where are all the cops going to be tied up for the next 10 minutes?
Time to call for more help. Got an animal control officer nearby? A constable serving papers? S.O. on the way to a transport?
Everyone's on different radios with different dispatches. That takes long minutes. They taught us in the academy that in the ghetto, for the worst run you can imagine, there's a good possibility that you're going to be "it", and have to handle it all by yourself, at least for 10 to 20 minutes, or until you see your OWN blood anyway.
I know what's constitutional, I know what's reasonable in most of the real world, and I know what's reasonable in certain other parts of the real world. Bottom line is, if you're one of the guys in that van, with most of the ghetto cops I know, you're not leaving that parking lot until the inside of the store is checked, OR until you're checked for weapons. I couldn't care less about the ID, I just want to know if there are guns under those circumstances. It may not be right, but I'd rather be suspended than dead, and a day at the beach with no pay isn't that bad a thing. I'm not saying it's right, I'm just saying that that's the way it is. (not saying that I would detain those guys in the van without a name for the guy who called on the phone, but many guys I know would, and I think it's reasonable).