Travel in France was disrupted hours before the Paris Games opening ceremony on Friday when arsonists set fire to three high-speed rail lines.

Numerous trains were canceled on Friday morning, and the railway company S.N.C.F. urged travelers to delay their plans, The New York Times reported. The fires started in towns nearly 100 miles outside of Paris and were set in pipes that hold cables used for signaling, according to S.N.C.F. Chief Executive Jean-Pierre Farandou. No one has claimed responsibility.

“The investigation is starting, I call on everyone to be cautious,” said French Prime Minister Gabriel Attal. “What we know, what we see, is that this operation was prepared, coordinated, that nerve centers were targeted, which shows a certain knowledge of the network to know where to strike.”

The arson attacks happened as French authorities have been ramping up security around Paris in anticipation of the Olympic Games, with around 55,000 police and soldiers monitoring the capital city, Reuters reported. Authorities expect to see more than 300,000 spectators gather near the River Seine for the opening ceremony.

Along with thousands of spectators having to change their plans, some Olympic athletes were also caught up in the commuting nightmare following the arson attacks. Two German athletes were forced to turn back after the rail line closures, and will likely miss the opening ceremony, according to the Associated Press, which cited a German news agency.

Police have also taken proactive security measures before the Games, targeting 155 people deemed potential threats and essentially placing them under house arrest, according to the Times. French authorities arrested a man in May for allegedly planning on carrying out an attack outside of an Olympic soccer match in the southern town of Saint-Étienne. A Russian man was also arrested on suspicion of attempting to “destabilize the Olympic Games.”

French Interior Minister Gérald Darmanin said that 5,000 people who had applied to work numerous different jobs during the competitions, such as security and stadium workers, were rejected when criminal records flagged them for “radicalization.” France has also requested the help of U.S. security. Officers from New York and Los Angeles, along with bomb-sniffing dogs, will be joining the security forces at this year’s Olympic Games in Paris, The Wall Street Journal reported.

Bruno Le Ray, a retired general who is in charge of Olympics security, said, “The security measures and the resources deployed for the Games’ opening ceremony are unprecedented.”