Authorities in London are investigating after small improvised explosive devices were discovered Tuesday morning at three separate locations near major transport hubs.

Police said the IEDs, which were found inside postal bags, “appear capable of igniting an initially small fire when opened.”

“The police are treating the incidents as linked, but are keeping an open mind regarding the motives,” London Mayor Sadiq Khan said in a statement.

One of devices, found around 10 a.m. local time at The Compass Centre near Heathrow Airport, was partially opened Tuesday morning after it was found by staff members, “causing the device to initiate” and catch fire, authorities said.

Officials evacuated the building as a precaution after the incident. No injuries were reported.

About an hour and a half later, British transport police responded to reports of a suspicious package in the mailroom at the Waterloo Station, a major rail and London Underground hub. Around 12:10 p.m. local time, authorities were called to investigate a third suspicious package reported at the offices of City Aviation House, near London’s City Airport. Neither package was opened, and no injuries were reported.

No arrests have been made in connection to the devices and no one has claimed credit for them. Authorities continue to investigate.

The official terrorism threat level throughout Britain is set at “severe,” indicating that intelligence analysts believe an attack is highly likely.

The Associated Press contributed to this report.