Washington State Patrol Trooper Kelly Spangler said a large section of a rock wall broke off the hillside and fell on a car just after 1 a.m., crushing the three people who were inside.
"These are very, very large rocks," she said.
No other cars were hit.
The interstate was closed to westbound traffic at milepost 70 for much of the day. It re-opened at 3 p.m.
Department of transportation engineers spent the day stabilizing the hillside where the rock broke free about two miles west of Snoqualmie Pass. They also removed overhanging loose rocks that were above the lanes on the highway, posing a potential risk to drivers.
The names of the three people who were killed have not been released.