I have read that is what they think may have happened. The truck driver and passenger, driver of car #1 (hit first) were all taken to the hospital, treated and released.
The road is a limited access, 4 lane toll road with grassy medians and no median barriers. The speed limit is 65.
Delaware said that after investigation, if warranted, they will charge the driver of the truck (who is a Maryland resident). So far, no charges warranted.
Delaware does have a seat belt law - driver's responsibility to be sure all passengers are belted in - none of the four girls were.
From what I’ve seen, the lack of seat belt use may not have been a major factor here. One of the five fatalities was belted in, so this was clearly a high-impact collision that would have caused a lot of damage even if everyone was belted in.