Moscow ? Russian investigators Tuesday launched a terrorism probe into a bomb blast that derailed an express train from Moscow to St. Petersburg, injuring more than 60 people traveling on one of the country’s busiest rail routes.

The explosion occurred late Monday near the city of Novgorod, about 300 miles northwest of Moscow. Investigators said they believe a homemade bomb placed underneath the tracks was detonated by remote control as the Nevsky Express train passed by with 251 people aboard.

The blast tore a three-foot wide gap in the rail line just 100 feet before a bridge. However, because the train was traveling at 80 mph, it had enough momentum to cross the bridge before derailing, Russian authorities said.

Though no one was killed, the blast unnerved a nation that has seen a lull in terrorist activity outside the volatile North Caucasus region in the last couple of years.

Nikolai Patrushev, director of the Federal Security Service, the KGB’s successor agency, told a national anti-terrorist committee meeting in Moscow that “the threat of extremism and terrorism has not been completely eliminated.”