Date: 201811
Local time of day: 1201-1800
Location: LAX.Airport, CA
Flight conditions: VMC
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight phase: Takeoff / Launch
ATC / advisory: Tower LAX
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Distraction; Situational Awareness; Confusion
Second reporter function: Pilot Flying; First Officer
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Critical; Ground Incursion Runway
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We were cleared to line up and wait on Runway 25R at LAX. Upon entering the runway; we were cleared RNAV DOCKR; cleared for takeoff Runway 25R. Takeoff Checklist was completed and power was applied. The 80 knot call was made and checked. At approximately 125 knots we realized that a plane appeared to be crossing at the far end of the runway. Judging our speed and rate of acceleration and the distance between our aircraft; the decision to continue the takeoff was made. In my judgement; this was the safest course of action not knowing the stopping distance that would be required and feeling confident in our ability to clear the aircraft if it did not completely cross the runway before we reached it. We executed a normal rotation. The aircraft exited the runway before we crossed its path on the runway by a margin of perhaps 75 feet. Several factors contributed to this event. First would be the either inadvertent clearing of an aircraft to cross a runway that was already in use for a takeoff; or a crew mistakenly crossing a runway without clearance. We heard no radio call clearing another aircraft across our runway. Another factor would be the time of day and weather conditions. [The] incident occurred approximately [half hour after sunset]. Adding to that was the weather. FEW000 was really more like patchy fog toward the far end of the runway towards the Pacific [Ocean] and the point of entry for the conflicting aircraft; making its detection more difficult.Continued emphasis on runway incursion events and there avoidance is certainly in order. The installation of red do not cross lights at runway and taxiway intersections would prove helpful though we would have been well past the runway lights when the crossing began; it would have proved helpful for the other aircraft.


Second narrative: On our second leg; after a 4+ hour sit at LAX and a confusing push back for a cross bleed start during a busy taxi; we were cleared 'on to hold' RWY 25R full length. While lining up with the runway; we were cleared 'RNAV DOCKR; RWY 25R; cleared for takeoff' which I read back and confirmed with the Captain that all items were completed and we were ready for takeoff. After the power was set and the 80 knot check; I noticed something moving down field in the incoming fog bank. At approximately 120 knot the [air carrier] crossing our runway became evident. The Captain elected to continue as he/we felt confident we could clear the crossing aircraft; the aircraft would likely be across by the time we got there and we would not be able to stop prior to where he was crossing. We rotated on speed and were approximately 100 feet AGL when we passed over the taxiway where Aircraft Y crossed; (taxiway P?). The tail of Aircraft Y jet had just cleared the runway when we went over. LAX Tower sent us to Departure before the gear were fully retracted so there was no discussion on the radio. Rest of flight was uneventful. There was a fog bank rolling in from the west along with dusk lighting from the setting sun which made visibility down the runway degraded. The light color of Aircraft Y jet blended with the color of the cloud bank well enough that the tail was the prominent visual cue that I could see when it came onto the runway. Had the Captain and I saw the jet sooner; a rejected take-off might have been in order. The degraded visibility and lighting was a factor in our delayed recognition of the intruding aircraft. I did not hear the word 'cross' at any time during the lineup or takeoff which leads me to believe Aircraft Y jet crossed our runway; [Runway] 25R; without clearance while we were on takeoff roll. I did not observe any lights on the crossing aircraft (wing or runway turn off lights). Further; the paint scheme of the aircraft was perfect camouflage for the conditions.Don't cross a runway without clearance; assuming that is what occurred. Training for Tower controllers if they cleared the jet to cross. Installing automated red crossing lights in the runway... additional vigilance during degraded vis; low light operations; by everyone.


Synopsis: B737 flight crew reported observing an aircraft crossing downfield during takeoff roll from LAX.