Red Bull were fined 50,000 euros ($58,300), with half suspended, for a grid breach before the start of Sunday's U.S. Formula One Grand Prix.
Stewards said a Red Bull team member re-entered the gate well area near the grid position of McLaren's Lando Norris after the formation lap had started and as marshals began to close the gate.
The reason for doing so was not disclosed in the statement.
Sources from more than one team told Reuters, however, that McLaren had placed a marker to help Norris line up his car correctly on the grid and the Red Bull employee had wanted to remove it.
The Briton qualified and finished second in the race, with Red Bull's Max Verstappen on pole position and going on to take the victory.
There was no official comment from either team, with Red Bull principal Laurent Mekies speaking to reporters before the stewards' decision and giving no details pending the outcome.