LAKE BUTLER, Fla. (AP) _ Evidence suggests that a tractor-trailer never braked before it rear-ended a car full of young relatives and shoved it into a stopped school bus on a rural highway, killing all seven children in the car, state troopers said.

As authorities tried to determine why the truck driver apparently didn't stop, the children's family tried to cope with unimaginable grief that grew even more heart wrenching after Wednesday's crash when a grandfather died.

William Scott, 62, ``had a massive heart attack tonight over all this,'' Barbara Mann, the children's mother and Scott's daughter, told CNN on Wednesday. ``I can't deal with it.''

Four of those killed were siblings _ three of whom were adopted _ and the youngest victim, a 20-month-old boy, was a foster child whose adoption into the family was to have been finalized Thursday. The two other victims were cousins; authorities had previously said all seven were adopted siblings.

Authorities were looking into whether the truck had a mechanical failure or whether its driver was tired or talking on a cell phone, among other possibilities, Florida Highway Patrol spokesman Lt. Mike Burroughs said. Also, a sample of 31-year-old Alvin Wilkerson's blood will be tested for alcohol, Burroughs said.

``We want to know why he didn't see a big, large school bus,'' Burroughs said.

The deaths were a shocking blow to this city of about 2,000 people about 50 miles west of Jacksonville.

``We're a small community, close knit and it's just been real hard,'' Union County Sheriff Jerry Whitehead said, struggling to keep his composure.

The children, ranging in age from 1 to 15, were the only people in a car that Whitehead said was turned to ashes in the wreck.

Two miles from home after school, the children were stopped behind the bus on the highway, where the speed limit is 60 mph, when the truck slammed into their vehicle. The car and the truck caught fire, and the bus ended up 200 feet from where it was struck, facing the opposite direction, Whitehead said.

``It was horrible. People were screaming, children were wandering around, two were laying (in) the middle of the road,'' said Joy Clemins, who lives nearby. ``It is like they were walking around in a dream.''

Wilkerson, the bus driver and all nine students aboard the bus were injured. At least six victims remained hospitalized Thursday.

Family and friends gathered Thursday around a campfire at the siblings' home at the end of a dirt road. A swing set sat empty in the front yard.

A woman who declined to give her name said relatives didn't want to talk Thursday. A sheriff's deputy then asked reporters to leave.

``The grief is unbearable and we just ask that you continue to pray for them,'' said Pastor Scott Fisher of the Lake Butler Church of Christ, which the family attends.

The car was being driven by a 15-year-old sister who had only a learner's permit and was at the wheel illegally.

``Even though she was an underage driver, it's my understanding she did not cause the accident,'' her aunt Tina Mann told CNN. ``The same thing would have happened had there been an adult in the car with her. We'd just have one more death in the family.''

Tina Mann said her niece had dropped off another child and was taking the children home to get ready to go to church.

The victims in the car were identified by authorities, friends and a pastor as driver Nicky Mann and her three adopted siblings, Elizabeth Mann, 15; Johnny Mann, 13; and Heaven Mann, 3.

Also killed were 20-month-old Anthony Lamb, whose adoption was to have been finalized Thursday, and Terry and Barbara Mann's nieces Ashley Keen, 13, and Miranda Finn, 8.

High school sophomore Tara Brown said Nicky Mann was her best friend and that she came to the school to pick up the children every day.

Three students from the bus were in serious condition and two were in good condition, hospital officials said Thursday. The bus driver was also hospitalized in stable condition.

The National Transportation Safety Board will investigate the accident.

Wilkerson had a valid commercial driver's license, according to the state Department of Highway Safety and Motor Vehicles. He was cited in 2000 for driving with a suspended license and was twice cited for operating a vehicle in unsafe condition, in 2000 and 2001.

Details were not immediately available, according to state officials.

The truck's owner, Crete Carrier Corp. of Lincoln, Neb., said it was cooperating with investigators.

``Our thoughts and prayers are with those involved in this accident and their families. We are in the process of gathering information, and at this point we are unable to accurately discuss the incident,'' said Jack Peetz, the company's executive vice president and chief operating officer.