Two adults and a 12-year-old boy are dead after a 20-vehicle crash on Highway 401 near Whitby on Friday.

Police responded to a call around 10:30 p.m. for the serious collision in a construction zone, which involved four tractor-trailers.

Five people are in critical condition and 11 others suffered serious injuries and have been take to hospitals across the region.

“This is a significant collision, a big scene,” OPP Sgt. Kerry Schmidt said.

Three people killed in this multi-vehicle pileup on the W 401 in Durham pic.twitter.com/Ot8OuGV2bc — Kris McCusker (@Kris680NEWS) October 3, 2015

“With all these vehicles crumpled up, it’s an unbelievable scene here with steel and aluminum twisted around from these vehicles that are not even recognizable as cars,” Schmidt said.

Investigators say the vehicles that the deceased were in ended up underneath the tractor-trailers.

“It appears that one of the transport trucks was travelling westbound and as traffic began to slow in front of it due to construction this vehicle did not slow down and continued right into the back of traffic,” Schmidt said.

Tractor trailer shown in image was vehicle that caused fatal crash after colliding into the back of multiple vehicles on Highway 401, Friday. CITYNEWS/Stephen Dagg

During the investigation, an OPP helicopter was capturing aerial shots of the scene.

The highway was closed in both directions between Brock Street and Salem Road overnight.

UPDATE:ROAD CLOSURE: #Hwy401 EB lanes are now OPEN at Salem Rd. #Hwy401 WB lanes remain CLOSED at Brock St. ^ag — OPP GTA Traffic (@OPP_GTATraffic) October 3, 2015

Schmidt said there were no environmental conditions and visibility was good at the time of the collision.

The investigation is ongoing but no charges have been laid.