But exactly what he knows remains a mystery.
Bostian, the 32-year-old train engineer, has told police he could not remember his speed, a law enforcement source told CNN.
Initial data showed the train was barreling into a curve at more than 100 mph before it derailed Tuesday evening, the National Transportation Safety Board said. That's more than twice the 50-mph speed limit for the curve.
Detectives later tried to question Bostian again, a police official said, but he refused to be interviewed -- and instead left with his lawyer.
But his lawyer has said his client has told police everything he knew.
So what do we know about Bostian?
He's an Amtrak veteran
Bostian has been an Amtrak engineer since 2010, according to his LinkedIn profile
. Before that, he was an Amtrak conductor for four years.
He graduated from the University of Missouri in 2006 with a degree in business administration and management.
A friend and former colleague described Bostian as a "great engineer."
"I mean, he was on point. If there was something he didn't know, he knew who to call to get the information," said Xavier Bishop, who was fired from Amtrak for attendance issues.
"I'm not going to sit there and say that we can't make mistakes, and we don't make mistakes. We're human. We all make mistakes, but, you know, for something like this? I don't know," he said. "There's just not enough pieces of this puzzle for me. Like something just doesn't seem right."
He took to social media after the crash
Hours after the train derailed, killing at least seven people and injuring more than 200, Bostian changed his Facebook profile picture to a solid black box.
Dozens of friends offered their support.
"Hey Brandon. Just wanted to say that we are all thinking about you, and you have our full support," one person wrote. "Every day, you will get stronger and stronger. And just remember that your Amtrak family is here for you Brother."
He tried to stop the train just before it derailed
The engineer applied full emergency brakes "just moments" before the train derailed, the NTSB said.
The train was traveling about 106 mph as it headed into a curve, where the speed limit was only 50 mph, NTSB board member Robert Sumwalt said. The speed limit immediately before the curve was 80 mph.
But even if investigators determine excessive speed caused the crash, it might not be the engineer's fault. It's possible there may have been a mechanical issue, such as faulty brakes.
He can't remember exactly what happened, his attorney says
Bostian's attorney, Robert Goggin, told ABC's "Good Morning America
" that his client "was beat up" in the crash.
"He has 15 staples in his head, stitches in one leg, the other knee is immobilized," Goggin said Thursday morning.
He added that Bostian suffered a concussion and doesn't remember why the train was going so fast.
"I believe as a result of the concussion, he has absolutely no recollection whatsoever of the events," Goggin told "Good Morning America." "I'm told that his memory is likely to return as the concussion symptoms subside."
Bostian's last memory before the crash was trying to slow down before the curve, his attorney said.
"Thereafter, he was knocked out -- thrown around just like all the other passengers in the train."
He seemed to love his job
After growing up in Memphis and attending college in Missouri, Bostian now lives in Queens, New York.
Neighbor Moresh Koya described Bostian as a responsible, nice guy who enjoyed his job.
"He liked it. He was happy working there," Koya said.
The St. Louis Post Dispatch photographed Bostian in 2007
, when he was an Amtrak assistant conductor.
On Facebook, Bostian is a fan of Railroad Humor
, a page "dedicated to the all men and women that make the railroad run and to those that have a sense of humor."
In September, he also posted about having to get up early for work.
"Not a morning person, but leaving the house at 6:30 a.m. does make for a neat scene on a comfortably brisk Sunday morning! (Had to get up early for work at 7:30 a.m. in Sunnyside Yard!)" Bostian wrote.