Australian authorities foiled an “Islamic-inspired” plot to bomb an airplane, the country’s federal police said Sunday.

Security at airports around the country has been ramped up since Thursday and several raids were carried out across Sydney Sunday morning. Four people have been detained but no one has been charged.

“In recent days, law enforcement has become aware of information that suggested some people in Sydney were planning to commit a terrorist act using an improvised device,” Andrew Colvin, the commissioner for the Australian Federal Police, said at a press conference Sunday. “We do believe it is Islamic-inspired terrorism. Exactly what is behind this is something that we will need to investigate fully.”

Prime Minister Malcolm Turnbull said the operation was launched to stop a terrorist plan to “bring down an airplane.”

“I can report last night that there has been a major joint counter-terrorism operation to disrupt a terrorist plot to bring down an airplane,” Turnbull said during the press conference. “The operation is continuing.”

Colvin added that authorities still don’t know the exact circumstances behind the alleged plot.

“At this time we don’t have a great deal of information on the specific attack, the location, date or time,” Colvin said.

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