A MKE Local Controller cleared a second aircraft for takeoff not observing the preceding aircraft on the same runway aborting takeoff.

Narrative

We were departing Runway 19R and landing Runway 25L in the very beginning of a departure and arrival push. I cleared Aircraft X off of Runway 19R; and then I taxied Aircraft Y into position and hold on Runway 19R. I specifically remember seeing Aircraft X going through Taxiway Mike then I looked back to see Aircraft Y who was still taking the runway and not yet in position and I cleared him for takeoff. As I was scanning Runway 19R I heard the CIC yell that Aircraft X had aborted his take off. I told Aircraft X to turn right on Taxiway Sierra and turn right on Taxiway Romeo and hold short of Runway 25L and told Aircraft Y to cancel his takeoff clearance. Aircraft X asked what the instructions were; I repeated them. Aircraft Y asked to clarify that I aborted his takeoff clearance. I said; 'Aircraft Y; affirmative; cancel your takeoff clearance.' He read it back. I asked Aircraft X for a reason for the aborted takeoff. He responded that it was because of an Engine Indicator Light. I have no idea how much time elapsed before the pilot decided to abort his takeoff and the time it took us to notice that he aborted his takeoff. When I looked at him going past Taxiway Mike he was going very fast and I was under the impression that he had either already started rotating; or was about to; which is why I cleared the next aircraft for takeoff. I have to wonder why the pilot himself didn't tell me that he aborted his takeoff. The only recommendation I can think of is having pilots that abort their takeoff rolls tell the controller sooner.