Transcript for 3 IEDs mailed to airports, train station in London

We move overseas to an explosive scare. A counterterrorism investigation is under way after three devices were mailed to airports and a train station in London. One caught fire. ABC's chief foreign correspondent Ian Pannell is at Waterloo station with the latest. Good morning, Ian. Reporter: Yeah, good morning, George. From Waterloo railway station as you say one of the three locations targeted with those letter bombs. Overnight police in Ireland joining counterterrorism forces here in the hunt for the London bomber. Three homemade improvised explosive device, three locations across London. One bomb discovered at the city's Waterloo railway station. The other two mailed to London's airports. One to the city airport. The other to the main heathrow and it was near there where someone started to open one of those letter bombs causing it to ignite. The building then having to be evacuated although happily no one was injured. Ian, the police pretty sure they're linked. Reporter: Yeah, that's right. The police are saying that they think it's linked and one of the key clues that they appear to have is at least two of those packages were mailed from Ireland which is why police there have joined the investigation. Counterterrorism command warning people in London to be vigilant this morning for any suspect packages that could be in the mail or even left at stations like this. George. Frightening stuff, Ian Pannell, thanks very much.

This transcript has been automatically generated and may not be 100% accurate.