LAKE BUTLER, Florida (CNN) -- News of a crash in which seven children perished so upset their grandfather that he had a massive heart attack and died, the children's mother said.
"I lost my daddy tonight," Barbara Mann said Wednesday. "My dad died of a massive heart attack tonight over all this. He lost all seven of his grandkids ... I can't deal with this."
William Scott, who was 62, The Associated Press reported, died at his home. (Watch as news of the deaths proved to be unbearable -- 1:18)
Alvin Wilkerson, 32, of Jacksonville, Florida, has been identified as the driver of the tractor-trailer which plowed into the children's car near Gainesville, Florida, Wednesday. The impact caused the car to slam into the rear of a school bus that had stopped to let children off.
The car burst into flames, killing all the children inside, Lt. Mike Burroughs of the Florida Highway Patrol said. (Full story)
They have been identified as: Cynthia Nicole Mann and Elizabeth Mann, both 15; Ashley Keen and Johnny Mann, both 13; Miranda Finn, 9; Heaven Mann, 3; and Anthony Lamb, 20 months.
All were adopted foster children, except for Anthony, who was in the process of being adopted, Burroughs said.
Ashley and Miranda were also cousins.
At the wheel was Cynthia Mann, who had a learner's permit. Under Florida law, it is illegal for a 15-year-old to drive without an adult. But Lt. Bill Leeper of the Florida Highway Patrol said that was not relevant to the crash.
"There are some questions we're going to need answered ... but those are questions we'll look at later," Leeper said. "Right now everything has to do with the truck."
Trucker had driving citations
Wilkerson has been hospitalized and is medicated, police said.
Initial estimates show Wilkerson's truck traveling between 50 mph and 65 mph when it struck the Manns' car, police said.
The tractor-trailer left no skid marks, police told CNN Thursday. But there were marks on the road indicating Wilkerson veered away only after the initial impact.
"Basically he just wasn't paying attention, couldn't slow down in time and just ran into the back of the other vehicle," Leeper said.
Wilkerson was cited in 2000 and 2001 for operating a vehicle in unsafe conditions and also for driving on a suspended or revoked license in 2000.
'Ready to go to church'
Cynthia Mann's aunt said the girl had just dropped off another child and was taking the rest of the children home "to get ready to go to church."
"It's my understanding she did not cause the accident," Tina Mann said of her niece. "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."
Three of the nine children aboard the bus were seriously injured and transported by helicopter to hospitals. None of the students' injuries were life-threatening, Leeper said.
A spokeswoman for Shands Hospital in Gainesville said eight patients were transported, ages 5 to 16. Two were in critical condition; three in serious condition, Betsy Miller said.
The driver of the bus was also injured, but her condition wasn't immediately known.
Meanwhile, the small Florida community where the victims lived reeled in shock.
A spokesman for the family, Scott Fisher, a minister, thanked the sheriff's department, rescuers and the public for an "outpouring of support."
"The family, of course, is deeply in grief," he said. "They have lost their children.
"We just cry with them, pray with them. And we let them know that this community is strong, that we are all here, that they're not alone. That it's not just this community, but the whole nation mourns with them," he said. "Tell them that maybe the children aren't really ours. They're God's, and He just called them home."