The death of a US student whose body was found in Rome's River Tiber is being treated as murder, his brother said after it emerged the teenager's bank card had been used following his disappearance.

Beau Solomon, a popular quarterback and University of Wisconsin-Madison student who arrived in Italy just hours before he vanished, was reported missing on Friday 1 July after last being seen in the early hours of the morning during a night out with his friends.

But his credit card was used after he disappeared, with thousands of dollars charged to the account.

Solomon's brother, Cole Solomon, 23, told the Milwaukee Journal Sentinel: "It's a murder investigation."

The body of the 19-year-old was pulled from the river on 4 July, and his parents are believed to have flown to Italy to formally identify him.

His university's chancellor Rebecca Blank said in a statement: "All of us at UW–Madison are greatly saddened by this loss.

"Beau was a bright and caring young man who lived the Wisconsin idea through his work at Badger Boys State and his desire to travel and experience other cultures. Our hearts go out to Beau's family and friends at this difficult time. UW–Madison is working with John Cabot University and with American and Italian authorities to assist Beau's family and to support the investigation into his death."

The school is not advising any of its students against travel to Italy as it said there is no indication of broader risks to the students in Rome or elsewhere.