police station mission help HELP!

you know the mission where you have to take the van and the electricians clothes and ram the real electrician off the road, then kill a certain cop at the police station?

well, ive done everything in the mission (got the van, got into the station, got the gun from the evidence room upstairs) BUT....which cop is the cop im supposed to kill? im supposed to kill a cop on the ground floor, but which one??