Schools, a public building and an airport were evacuated in two Serbian cities on Monday morning following several bomb threats.

The Education Ministry confirmed that dozen of schools in Belgrade were evacuated across the Serbian capital. A press release added that the police searched 97 schools and found no explosive devices. “Students returned to classes in some schools or were sent home in schools where searches are ongoing,” it said, adding that the false bomb threats were sent as early as 6:15 am.

The Beogradjanka building in downtown Belgrade was also evacuated while police and firefighters searched it. That 23-floor high-rise is home to shops and offices. Security guards told N1 that the bomb threat was sent to a private school in the building, forcing evacuation.

Police and firefighters also search the Konstantin Veliki airport in the southern city of Nis which also received a bomb threat.

Schools, shopping malls and other public buildings have received bomb threats on several occasions over the past two months with some sources claiming that the threats have come from Ukraine because of the Serbian authorities’ failure to impose sanctions on Russia over its invasion of that country.