London police this week are investigating incidents in which small explosive devices were delivered to the offices of three major transportation centers in the metropolitan area, including two airports and one train station.

London's Metropolitan Police Service said that the devices, which were delivered on Monday, "appear[ed] capable of igniting an initially small fire when opened."

This was proven to be the case after a Heathrow staff member unsealed one of the packages, delivered to Heathrow's offices near the airport grounds at 9:55 a.m. local time.

Part of the package burned when the device went off, and no injuries were sustained by the person who opened it or by any surrounding personnel.

While the office was evacuated, no incoming or outgoing flights were affected by the incident.

Later that day, another device was reported at the Waterloo Station mail room around 11:40 a.m. The last was found at City Airport, a local commercial air transit hub, before noon. Neither of the packages was opened.

According to Britain's Office of Rail and Road, approximately 94,355,000 passed through Waterloo train station in 2018, making it the most popular train station in the city.

Heathrow, the largest out of five airports in the London metropolitan area, is similarly popular when it comes to foot traffic, with roughly 213,668 passing through its terminals and halls on a daily basis.

Photos obtained by Sky News correspondent Alistair Bunkall showed that the packages sent to Heathrow's office were sent with stamps designed with small hearts. The stamps were issued by Ireland's post office for Valentine's Day 2018.

The incidents are being investigated as "a linked series," according to a statement made by the U.K.'s Metropolitan Police Counter Terrorism Command, and they are "keeping an open mind regarding motives."

So far, no individual or group has claimed responsibility for sending the incendiary devices, and no arrests have been made.

Since the discovery of the packages, Scotland Yard has warned transit stations throughout the U.K. "to be vigilant," and London Mayor Sadiq Khan has asked London residents to do the same.

Irish police announced that they will be working with British counter-terrorism police on investigating the matter, though no other details have been given on the nature of the collaboration.

Transportation centers in the U.K. aren't the only ones that have been affected by the discovery of the devices. New York Gov. Andrew Cuomo has put travel hubs in the New York metropolitan area on high alert.

"The explosives found in London transportation hubs are stark reminders of the threats we face today," Cuomo said in a statement. "In the wake of this act of violence, New York stands with Britain and all our allies against terrorism in all its forms."

This article originally appeared in IBTimes US.