A plane that took off from Reagan National Airport on Monday morning had to make a quick landing at Dulles International after its pilots reported smoke in the cockpit.

Alaska Airlines Flight 1, headed to Seattle, had just taken off around 10:22 a.m. when the pilots “noticed indications of smoke” in the cockpit, said Cole Cosgrove, a spokesman for the airlines. The plane landed at Dulles at 10:38 a.m.

Cosgrove said he did not know if the crew saw smoke and/or smelled smoke. It was also not known where the smoke was coming from immediately.

Chris Paolino, a spokesman for the Metropolitan Washington Airports Authority, said six people were taken to an area hospital. He said they were all members of the plane’s crew.

The Alaska Airlines Boeing 737 was carrying 161 passengers, all of whom were evacuated safely at a gate at Dulles and rebooked Monday and Tuesday on other flights, officials said.

Authorities said the cause of the incident is under investigation.