The Ukrainian Security Service has asked international organizations to help investigate Friday bomb attacks on the eastern city of Dnepropetrovsk, the Security Service’s First Deputy Chairman Vladimir Rokitsky said, according to RIA Novosti.

Four bombs went off in Dnepropetrovsk on Friday afternoon within an interval of one and a half hours, injuring at least 29 people, according to official data. Unofficial reports put the number of blasts at 10. A criminal case has been opened under a terrorism statute.

“The [Security Service’s] Antiterrorism Center and its head have turned for help to international organizations that have an opportunity to assist in investigating this crime,” Rokitsky said in an interview with the First National Channel late on Friday, without elaborating on which organizations had been contacted.

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The Russian and Polish special services have already offered their help, he added.

He also raised doubts about whether the attacks should be qualified as acts of terrorism.

“I would not immediately declare that those were terrorist attacks,” he said. “Let us understand first how this crime should be qualified.”

More than 20 people were hospitalized with fragmentation wounds following the attacks, emergencies services said. Four of them are in a grave condition. At least 10 minors were among those injured.

The bombs exploded at a tram stop in central Dnepropetrovsk, outside a cinema, near a railway terminal and in a public park. All of the explosive devices were placed in garbage bins.

No one has claimed responsibility for the attacks. President Viktor Yanukovych described them as “yet another challenge for us, for the entire country.” He is expected to visit Dnepropetrovsk on Saturday, his press service said.

Security has been strengthened across the city, and army units have been deployed to patrol the streets. All mass events in the city have been cancelled until May 2.