RELATED: See pictures of the rescued kittens
On Tuesday, the Pennsylvania SPCA arrested driver Bill Mitchell. He was given three citations, one for each kitten.
"There are three summary citations. Upon conviction he could pay a $750 fine for each conviction," said George Bengal of the PSPCA.
Authorities don't know how the kittens got on the bus, only saying they were in a bag. Investigators say the bus was not moving at the time.
Witness Terri Gerretz said Mitchell showed no remorse about tossing the kittens.
"He waited five minutes, but he was more interested in getting to his next run," she said.
Gerretz said she could hear the kittens hit the ground after they were thrown.
"My windows are thin, but you could hear a smack," she said.
Gerretz and her daughter then ran outside, collecting the kittens in a bucket with a blanket as they called authorities.
The bus company, Atlantic Express, said Mitchell has never been trouble in his four years with the company but acknowledged if the allegations are true, they're troubling.
The kittens are recovering at the PSPCA. They'll likely remain there for a week, then be put up for adoption.
For more information about the PSPCA, visit their website or call (215) 426-6300.