The second crash, on the Sofia-bound carriageway, was likely also caused by the fog, with drivers also distracted by the first crash, BTA reported an official as saying.
The Trakia motorway runs from west-to-east of Bulgaria, and connects the capital Sofia to the Black Sea coast.
Road administration director Galin Kostov added that speeds were unsuitable for the conditions, which included wet road surfaces, and the motorway would remain closed until it had been cleared, BTA reported.
Yambol regional governor Georgi Chalakov said reports first came in at 07:10 local time (05:10 GMT), with the injured taken by ambulance to hospitals in the Yambol, Sliven and Burgas regions.
Chalakov told BTA rescue teams had to remove sections of the crash barrier to reach people. He added that the fog meant that helicopters could not be used in the evacuation efforts.
Source: www.bbc.co.uk
