A mystery drone was spotted flying over a Belgium nuclear facility on Saturday, local authorities said, a day after one of the plant’s reactors came back on line after a four-month closure caused by sabotage.

The mystery appearance by an unmanned aircraft, on which Belgian authorities refused to provide much detail, resembles a spate of similar drone sightings over nuclear plants in neighbouring France this autumn.

Around 20 unidentified drones have been spotted over nuclear plants since October throughout France.

Belgium worried by mystery drone flying over nuclear plant

“We can confirm that the East Flanders prosecutor’s office has opened an investigation into a drone flight over the Doel nuclear plant,” a spokesman for the investigation told Belga news agency.

“We will not provide further information for the time being,” the spokesman added, hours after the plant’s operator, GDF-Suez unit Electrabel, first disclosed the incident, which took place early Saturday.

The imposing Doel nuclear site sits on a riverbank near the North Sea about 25 kilometres (15 miles) north of Antwerp. It holds four of Belgium’s seven reactors.

One of those reactors, Doel 4, was shut urgently in August after a leak, caused by tampering, gushed out 65,000 litres of oil lubricant.

A steam turbine weighing 1,700 tonnes was severely damaged by the loss of lubricant, requiring a 30-million-euro ($37-million) repair job that was carried out in Germany.

Belgian prosecutors have refused to confirm the sabotage as an act of terrorism, without excluding it either.

Two other Belgian reactors still remain shut, both due to cracks in their reactor containment vessels.

To face a potential shortfall in power, Belgium’s government on Thursday said plans to close the Doel site’s two oldest reactors, both nearly four decades old, would be suspended.

The Doel 1 and 2 reactors were due to be the first to be shut as part of Belgium’s planned phase out of nuclear power by 2025, but the government said it would seek to keep them operating.

Nuclear plants account for 55 percent of Belgium’s power generation, and the loss of over half of the country’s nuclear power has caused concerns of a shortage or even a blackout.

Regulators have put in place special measures to help meet Belgium’s needs this winter.