A man, 37, and a woman, 25, were killed in the June 19 night-time collision on the lake, a popular tourist destination.
Brescia Prosecutor Francesco Prete told state TV on Monday that the suspect is being investigated for two counts of manslaughter. He was also jailed for failure to help the stricken boaters.
“The worst thing was he went away and didn't lend assistance,” Prete said.
Investigators alleged that the suspect was one of two German tourists from Munich whose motorboat slammed into the Italians’ smaller boat. The man was found dead in the badly damaged boat, while the woman’s body was found two days after the crash during a search of the lake.
After the accident, the Germans returned to Germany.
The man who voluntarily returned to Italy had driven the motorboat, and he was accompanied by his wife to the Italian border early Monday, where his lawyer and police awaited him, the LaPresse news agency said. It said the man returned to Italy after learning a European arrest warrant had been issued for him.
The two men had been free immediately after the accident while the investigation continued. Authorities were analyzing blood tests to determine if they had been drinking before the crash.
Police located the tourists after people noticed their docked motorboat was damaged, according to Italian media.
Corriere della Sera has reported that investigators found pieces of wood from the Italian man’s boat embedded in the tourists’ boat.