The girl, Chloe, was with her grandfather in a play zone on the ship with a wall of glass windows, attorney Michael Winkleman said.
Chloe enjoyed banging on the glass at her brother's hockey games and Winkleman said her grandfather took her over to what he thought was a glass wall.
"He thinks it is all glass, there is a wood railing right there, he puts her up on there thinking that she is going to bang on the glass and it's going to be great and she goes to bang on the glass and the next thing he knows, she's gone," the attorney said.
Winkleman called the incident aboard the Royal Caribbean's Freedom of the Seas ship a "preventable tragedy." The 11-story fall occurred while the ship was docked Sunday in San Juan.
The girl's father is a police officer in South Bend. The family plans to stay in Puerto Rico until the investigation is complete.
Police Sgt. Nelson Sotelo said the toddler's maternal grandfather, Salvatore Anello of Valparaiso, Indiana, is under investigation. He said the rest of the family is from Granger, Indiana.
The police chief of South Bend, Indiana, Scott Ruszkowski, confirmed to WSBT television that the girl was the daughter of Officer Alan Wiegand, and the department issued a statement saying it "asks the community to pray for the entire Wiegand family as they grieve and to respect their privacy."
Ruszkowski disputed the statement issued by Puerto Rico police that the girl was being held out of a window.
Royal Caribbean Cruises called it a tragic incident and said it was helping the family. A spokeswoman said the ship departed Puerto Rico on Monday for St. Maarten on a new itinerary and declined further comment.
The Associated Press contributed to this report.