Italy's antitrust authorities are going into combat over Rome's Colosseum, opening an investigation into ticketing operations that have seen tourists paying inflated prices to visit the ancient Roman arena where gladiators once fought. Italy's most popular tourist venue, the Colosseum attracts millions of visitor each year. But tourists complain it is often impossible to find regular tickets as big tour companies scoop them up on the official website weeks in advance and then repackage them as much more expensive guided tours.