Counter-terrorism officers are "keeping an open mind" in the hunt for those responsible for sending explosives to three transport hubs in London.
Devices were found in A4 postal bags at London City Airport, Heathrow Airport and Waterloo Station on Tuesday.
Irish police are assisting with the investigation as Republic of Ireland stamps were on at least two packages.
British Transport Police said it had more officers on the transport network to provide "reassurance".
Explosives found at airports and station
Both airports and Waterloo station are running as normal, with London City Airport saying there would be a "visible police presence" in the area.
Nobody was injured by the "small improvised explosive devices", which were found in white postal bags containing yellow Jiffy bags.
The first package caught fire when it was opened by staff at Heathrow's Compass Centre at about 09:55 GMT.
The other two were discovered over the next three hours.
The Met said all three appeared capable of igniting a small fire when opened.
The packages sent to Heathrow and Waterloo had stamps issued by the Irish postal service for Valentine's Day 2018, featuring a heart motif and the words "Love Eire N".
The senders' addresses on both were also given as Dublin.
The Gardaí confirmed it was assisting the Met with its inquiries.
Met Police Deputy Commissioner Sir Stephen House said the investigation was in its "very early days".
Speaking at a London Assembly police and crime committee meeting, he said advice had been issued to London's transport hubs to be "more vigilant than they normally are" for suspicious packages.
While the devices did not affect air or rail services, workers were evacuated from City Aviation House at City Airport and Heathrow's Compass Centre, where the packages were sent.
Both have since reopened although workers have been told to be vigilant for further suspicious packages.
Transport Secretary Chris Grayling urged people to report "anything suspicious" to police.
Mayor of London Sadiq Khan added: "Our thanks go to police, security, transport staff and all involved for their swift actions to keep our city safe."