BRUSSELS—The Belgian government has been stripping or blocking the security clearances of people working at the Brussels airport because of past connections to people who left for Syria, possibly to join extremist groups fighting there, officials said.

Belgian authorities grew concerned about the threat when they discovered that someone who worked at the airport had left for Syria in 2013, officials here said. That prompted the authorities to begin scrutinizing the associations of people working at the airport more closely.

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