https://www.bbc.com/news/uk-england...r&ns_linkname=news_central&ns_mchannel=social
The "small improvised explosive devices" were found in A4 postal bags, the Metropolitan Police said.
The Met's Counter Terrorism Command is treating it as a "linked series" and "keeping an open mind regarding motives".
No-one has been hurt, the force added.
The Compass Centre in Heathrow's grounds was evacuated after a package was reported to police at about 09:55 GMT.
The device set alight when staff opened the bag.