Date: 201508
Local time of day: 1201-1800
Location: SEA.Tower, WA
Flight conditions: VMC
Light: Daylight
Aircraft: Medium Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower SEA
Second aircraft: Large Transport
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Distraction
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Rejected Takeoff
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: Aircraft Y was instructed to line up and wait and was told that traffic would cross downfield. Aircraft Z was given a crossing clearance across RWY 16L at N and told that traffic was holding in position. Immediately thereafter aircraft X was given a crossing clearance and told of the holding traffic. I then assigned visual separation to the preceding departure; changed them to departure; and then scanned my line-up for strip accuracy and potential crossing holes. I then looked up at aircraft Y; scanned the runway; which was clear; and gave aircraft Y his take-off clearance. I did not see that aircraft X had not crossed the runway yet; and after my scan down the runway they started their crossing. At which point aircraft Y said they saw the red lights and were aborting their take-off for traffic crossing in front of them.The only changes that I would make are based on my actions. There were contributing factors at play that I did not take as seriously as I should have.


Second narrative: While taking the runway for takeoff; we were told to 'line up and wait; traffic crossing downfield.' I heard Aircraft Z 'cleared to cross' (but no one else; even though the Captain thought he might have heard a second aircraft cleared to cross). As Aircraft Z crossed the runway; the Runway Advisory Lights illuminated red and I made the comment; 'red means stop;' a reference to the [manual]. As Aircraft Z exited the runway; the red lights extinguished and we were cleared for take-off. I read back the takeoff clearance and diverted my attention to the engine indications as the captain selected TOGA. When I looked away from the instruments to the runway; the runway advisory lights were illuminated red. Simultaneously; as the captain aborted the takeoff; I saw what I thought to be an [large transport] crossing the runway at the departure end. We queried tower as to whether or not he/they were aware an airplane had entered the runway after our takeoff clearance; but did not receive a response. We taxied back around and took off. Tower thanked us for managing the 'problem' as he sent us to the subsequent departure frequency. I think this was a failure on the tower controller's part; either he did not realize an airplane was crossing an active runway; or he did not realize an airplane he had cleared to cross an active runway had not done so in a timely fashion; then subsequently cleared us for takeoff.


Synopsis: Controller and pilots report of being cleared for takeoff and an aircraft that was cleared to cross the runway did in a slow manner. Controller thought aircraft had already crossed and then cleared the carrier. Carrier started takeoff roll and observed red lights down field and aborted takeoff.