Can bees fly downwards?

a swarm of bees have made their way into my house - the house was fully locked up (no open windows, doors or visible signs of entry). the only entry i can think of is via the chimney but this would require them to have flown downwards. the vast majority of the literally 1000's of bees were dead or dying when discovered and were close to the window in the room - looks like they were trying to fly out there