The plane, which belongs to Austrian Airlines, was heading to Vienna, Austria.
"In line with our protocol, an air force fighter jet took off and escorted the aircraft until it landed at Sofia Airport," Bulgaria's transport and communications authority said, according to Sky News.
In a statement, Austrian Airlines confirmed that a threat had been made against the plane and that it was being searched.
Six crew members and 51 passengers were reportedly on board. All were unharmed and safe, an airport spokesperson said, according to Israel's The Jerusalem Post.
The plane was taken to an isolated location, where it was examined by bomb technicians and security personnel, according to the newspaper. No explosives were found initially.
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