SHANGHAI—The passenger ship that capsized on the Yangtze River with more than 400 people on board was designed for river conditions and had been in service for more than two decades, according to state media and naval experts.

Authorities Tuesday were investigating but had yet to explain why the Eastern Star, or Dongfang Zhixing, overturned amid bad weather Monday night and why the captain and the chief engineer left the ship before evacuating the other 454 people on board.

Weather has been cited as a possible cause. China’s Meteorological Administration confirmed a tornado struck the area amid heavy rains and winds in Jianli county, where the accident occurred.

The Eastern Star was built in 1994, according to China’s official Xinhua News Agency. It said the ship has a length of 76.5 meters and was designed to carry 534 people—more than the numbers on board.

Apostolos Papanikolaou, director of the ship design laboratory at the National Technical University of Athens, said the age of the ship shouldn’t have been a factor. Ships sailing in rivers and lakes are built to meet different conditions than those plying open seas, he said, and safety regulations are less stringent on the assumption of calmer waters and closer proximity to shore in an emergency. “The danger of a wave flooding the deck is not the same as in open seas,” Mr. Papanikolaou said.