In the tunnel, why is there a stopped truck and why does John McClane shoot the people in it?

Question

See this summary from Die Hard Wiki about Die Hard with a Vengeance:

McClane has Jerry drive him to the tunnel as Zeus went to Yankee Stadium. As Jerry was driving, he gives McClane a tour of tunnel 3. Then they stopped when they notice a truck blocking their path. Jerry honks his horn and the truck still doesn't move. McClane finds this suspicious and decides to investigate. He takes Jerry's hardhat and jacket, disguising himself as an aqueduct security guard and makes his way to the truck. Inside the truck, Nils and Klaus wait for McClane to get close enough so they can kill him. Unfortunately for them, McClane beats them to the draw and fires several shots into the door of the truck, killing them both.

While watching the movie, I didn't understand what was the role of this truck and why did it stop? And why does John McClane kill the drivers without identifying them?

We can assert that the terrorists' trucks were the last to enter in the tunnel, so John McClane thinks that this truck can only be one of the terrorists. But I think this assumption is weak. What if it was a normal driver in the truck?

Answer 1

What was the role of the truck?

A rear guard. It's a good tactic to keep a lookout or a small force at the rear. That way they make sure nothing interferes with the operation and no one else drives into the pipe or decides to poke around (like John indeed does). This is mentioned in the movie, when the bad guys try to contact the truck:

Targo: We are coming to the dam. You can call the rear guard.

Simon: Rear guard, you can close up now. We've reached the dam. You can come up now. Nils!

Why does John kill them without identifying them?

He did take a risk, but it was an informed guess. Here's the dialogue before the scene:

John: Any extra dump trucks come through here the last couple minutes?

Foreman: I'm writing those fu@%ing clowns up! They better start paying attention to work orders.

John: What happened?

Foreman: A dozen idiots tore a$$ up the tube. We're not loading up there anymore, we're loading over here!

John is looking for a convoy of dump trucks, and a dozen of them just sped through the aqueduct, even though they're not supposed to do that. It'll be some coincidence if one of them happened to be some innocent driver in an actual dump truck who forgot the instructions, joined the robbers' convoy and then stopped in the middle of the pipe for no reason and didn't answer John when he repeatedly called out at them. John relied on his instincts as a cop, and the movie justified them: If he would've come any closer, they would've shot him through the door.