CHATTANOOGA, Tenn. (WIAT) — “I been calling up to the school. Since school started, letting them know my kid said he’s drinking on the bus,” Jasmine Mateen said.

Mateen’s three children were on the school bus that crashed Monday in Chattanooga. One of them was killed.

Tuesday, officials at Children’s Hospital said six students are in the ICU in critical condition while six others are stable in regular room. They explained it was particularly difficult to identify the injured children, as they were all wearing uniforms and of course had no ID on them. Officials say most of the kids were too dazed when they arrived to talk, or so young that when asked the name of their mother, they only knew ‘mama.’

We’re also learning more about the driver of the bus. Mateen says she warned the school that the driver would allegedly drink and drive.

At this hour, federal investigators are in Chattanooga, working to determine exactly what caused the crash that killed five children and injured more than two dozen others. Police are also investigating. They arrested the driver, amid suspicions he purposely caused the crash.

An arrest warrant for the driver, identified as 24-year-old Johnthony Walker, says he was driving well over the 30 mph speed limit on a winding, narrow road.

The bus Monday afternoon was carrying 35 Woodmore Elementary School children when it flipped over on its side and wrapped around a tree, killing one boy and four girls. Three of the victims killed were in fourth grade, one was in kindergarten and one was in the first grade. Mateen lost one of her three kids that were riding on the bus at the time of the crash. She made serious allegations to CBS News.

“My daughter said, right before the bus flipped, that he was speeding around the curb, and asked them, are you all ready to die?” Mateen said.

Police arrested Walker and charged him with five counts of vehicular homicide.

“Just last week, he told the kids, shut the “f” up, I got a hangover,” Mateen recalled.

It took hours for emergency crews to remove all the children from the mangled debris.

“The most unnatural thing in the world is for a parent to mourn a loss of their child, there are no words that can bring comfort to a mother or a father and so today a city is praying for these families,” Chattanooga mayor Andy Berke said.

NTSB investigators will be looking at the camera on board the bus, hoping it will help them figure out exactly what happened. Meanwhile, more charges could be pending against Walker.

The driver was also involved in another school bus collision two months ago. Little details on that crash have been released.