2 London airports and its busiest train station were sent makeshift bombs at the same time

Jack Taylor/Getty Images

A general view of aircraft at Heathrow Airport in front of the London skyline.

Police in London have launched a terror investigation after makeshift bombs were sent to two of the city's airports and its busiest train station on Tuesday.

The Metropolitan Police said that the items sent to Heathrow Airport, London City Airport and Waterloo Station were "small improvised explosive devices," Sky News reported.

In a statement, the police said they are treating the incidents as "linked." Trains and flights were not delayed.