The girl -- who was traveling alone aboard United flight 1695 from Seattle to Newark -- awoke from a nap to find fellow passenger Vijakumar Krishnappa's hand on her thigh, according to a criminal complaint filed against him in federal court in New Jersey.
Krishnappa removed his hand and she went back to sleep, but soon she was "startled awake" again. Krishnappa had his hand on her groin, "rubbing the victim through her clothing," according to the complaint. He also "tried to put his other hand inside of the victim's pants," the complaint claims.
"She was horrified," her attorney, Johnny McCray, told ABC News. "It's affected her ability to sleep and concentrate."
The girl notified a male flight attendant, who reportedly told her "that's not cool" and moved her to another seat, McCray maintains.
The flight attendant "did appear to have somewhat of a cavalier attitude," McCray said.
According to the attorney, it was the girl's mother -- not United -- that contacted the authorities after the plane landed. Meanwhile, Krishnappa had departed the airport.
McCray claims the airline violated its own policy, which he says directs employees to immediately notify authorities following allegations of a criminal act committed in the air.
"The safety and security of our customers is our top priority," United said in a statement. "We take these allegations seriously and continue to work closely with the proper authorities as part of their review."
Because United's policy only covers unaccompanied minors age 15 and below, the airline "assumes no financial or guardianship responsibilities beyond those applicable to an adult passenger," according to its contract of carriage.
Asked to comment on both policies, United told ABC News, "we are also reviewing the way that this situation was handled on-board and after the plane landed in New Jersey."
Krishnappa was later arrested, released on $50,000 bail, and forbidden from contacting the girl. His attorney did not immediately respond to ABC News' request for comment, but told the Washington Post his client "adamantly denies the charges and deserves to be considered an innocent man."
"Our principal concern is making sure that Dr. Krishnappa is prosecuted to the fullest extent of the law," said McCray, adding that the family has not yet discussed suing the airline.
In 2015 the FBI told ABC News it believes in-flight sexual assaults are vastly under-reported and estimated at least one incident reported at least once a week in New York City area alone.
ABC News' Aaron Katersky, Josh Margolin, and Whitney Lloyd contributed to this report.