From the Washington State Patrol:

Interstate 5 was shut down for 11 hours overnight after a fuel tanker carrying 1,000 gallons of diesel and 10,000 gallons of gasoline overturned southbound near Federal Way.

The incident began around 10:15 p.m. Tuesday, when a disabled semi stopped on the right shoulder and was hooking up to a tow truck. A second semi stopped behind it to provide assistance. As the second semi pulled off of the right shoulder into the right lane, it struck the fuel tanker, causing it to roll onto its side. The threat of an explosion and the debris on the roadway initially caused all but the HOV lane to close to traffic. Around 1:30 a.m. Wednesday, the HOV lane closed and all traffic was diverted off of the freeway onto State Route 99.

Several fire departments from around King County, the Department of Transportation and the Department of Ecology were on scene throughout the night assisting with containment of the spill.