An investigator speaks on the phone inside a supermarket after an explosion in St.Petersburg on Wednesday. (Dmitri Lovetsky/AP)

A bomb exploded in a supermarket in the Russian city of St. Petersburg on Wednesday, injuring 10 and raising new concerns of terrorism days after Russian authorities claimed that a tip from the United States helped ward off a major attack in the same city.

The bomb, which authorities called a "homemade explosive ­device" with the strength of about half a pound of TNT, exploded in locker at a Perekryostok ­supermarket in the northeast of St. Petersburg, Russia's second-

largest city and the country's cultural capital. The bomb was packed with what authorities called "killing agents," which local reports said were ball bearings.

Russian authorities did not immediately call the explosion a terrorist attack, classifying it instead as the attempted murder of two or more people, without naming the intended targets. No information was released about a suspect.

Nine people were hospitalized, including one in critical condition with shrapnel wounds to the stomach. As of Wednesday evening, none of the victims had died.

Local reports said that the explosion knocked down the door to the supermarket and left a hole in the wall.

The attack came 10 days after Russian President Vladi­mir Putin called President Trump to thank him and CIA Director Mike Pompeo personally for a tip from the CIA that thwarted a planned terrorist attack in St. Petersburg.

According to a White House readout of the call, the tip allowed Russian law enforcement to track down a group of suspects planning to bomb the Kazan Cathedral in the city center, among other targets, attacks that "could have killed large numbers of people."

The call was unusual. While countries often share intelligence, their presidents rarely thank one another for it publicly. Both Trump and Putin have said that combating terrorism could provide a shared platform to improve strained relations between Russia and the United States.

St. Petersburg, a sprawling ­metropolis with a downtown renowned for its czarist-era cathedrals and palaces, has recently been a target for terrorism. In April, a suicide bombing by a naturalized Russian citizen from Uzbekistan killed 16, including the bomber.

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