LONDON — Three suspicious packages that appeared to contain homemade bombs capable of igniting a small fire were found Tuesday in and around transport hubs in London, the authorities said.

The three packages were described as similar: all midsize white envelopes with padded manila envelopes inside.

The London Metropolitan Police said that a counterterrorism unit had been called in, and was treating the incidents as linked, but that it was “keeping an open mind regarding motives.”

No one was injured, but the police issued warnings for people to be vigilant around city transportation hubs.

“These devices, at this early stage of the investigation, appear capable of igniting an initially small fire when opened,” a police statement read.

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