In July, a surveillance camera caught a man throwing an incendiary device onto the building that damaged part of the roof.

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"They x-rayed the cylindrical shell, which was about 10 inches (25cm) and confirmed it was a live incendiary device, " a spokesman said.

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They try to cover their tracks after handling explosive material by disposing of any shoes and clothes worn while putting the incendiary device together.

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Kennedy library, but police said later it was a fire that may have been caused by an incendiary device, and it was not clear whether it was related to the bombings.

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She said officers also found a blast incendiary device in the front passenger foot well of the van, that police suspected was intended to be used to destroy forensic evidence from the van following the deployment of the mortars.

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