Published: Jan 22, 2013 11:06 PM EST
Updated: Jan 23, 2013 12:27 AM EST

COLLIER COUNTY, Fla - Many questions remain as to why cable barriers along Interstate 75 snapped during a crash that killed a mother and her two daughters Monday morning.

Cecilia Douglas and her daughters Madison and Rylee all died after their car went through the fence and into a nearby canal.

The barriers are designed to prevent deadly wrecks. Now the Florida Department of Transportation is looking into what went wrong.

In 2004, FDOT spent more than four million dollars to build the cable barrier system along parts of Alligator Alley and other Florida interstates.

"We saw fewer crashes occuring," says FDOT Spokesperson Debbie Tower. "It was catching vehicles and preventing them from getting underneath that wildlife fence and then into the canal."

But, Monday morning, that barrier did not stop Cecilia Douglas and her two daughters from plunging into the canal.

A passerby frantically called 911. She told dispatch, "There's a a huge fence. I don't even know how it went through the fence because there's not an opening."

The system is designed to set off a strobe light, so law enforcement can quickly find the scene. It is also supposed to send a signal to the Regional Traffic Management Center. Monday, both the strobe light and signal worked.

"It's not an absolute, but by and large the cable barrier system has been extremely effective," says Tower.

Now FDOT officials are left to investigate how the vehicle got through. "We need more information frankly from law enforcement about the incident and we need to take a look at the system itself as we repair it to have more information," says Tower.