MILAN – An electrical short circuit is being investigated as the probable cause of a fire in the international terminal at Rome’s Fiumicino airport that forced the cancellation of dozens of flights, authorities said Thursday.

Vito Riggio, president of Italy’s civil aviation authority, ENAC, told a news conference that the investigation of the fire at Italy’s busiest airport was continuing, but that the cause was “probably a short circuit in the commercial area.”

The fire forced the closure of the airport for much of the day, with only 19 long-range international flights permitted to land in the morning. Departures resumed in the afternoon and were slowly returning to normal.

Authorities initially said the fire did not appear to have been intentional, but the news agency ANSA reported that prosecutors were investigating the possibility of arson.

“We need to understand if what happened last night is just an accident or if there are problems that need to be identified,” Riggio said at a briefing, according to ANSA, adding that investigators were looking at why the fire spread so quickly.

Some 450 square meters (more than 4,800 square feet) of retail space was involved in the fire, which broke out just after midnight and was brought under control about five hours later, authorities said. Three people suffered light smoke inhalation.

Damage was contained to the commercial area located beyond the security check-in, said the head of Rome’s airport, Lorenzo Lo Presti.

Fiumicino is Italy’s busiest airport, with some 130,000 passengers a day. The international terminal remains closed, and flights were being reassigned to other terminals.