Six explosive devices have been sent through the post to homes in Luton.
The devices were in A4-sized brown envelopes which contained a padded bag and a plastic container with lighter fluid inside.
Although some of the envelopes were opened, none ignited, but police say they could have caused injury.
Police on Saturday then carried out further searches of the Luton and Leagrave postal sorting offices and discovered another four packages.
Bomb squad
Bomb squad experts successfully removed the packages.
Further searches of the sorting offices were carried out on Sunday but detectives said there seemed to be no obvious motive behind the sending of the packages.
Det Sup Nicky Dahl said: "At this stage there appears to be nothing to connect the households which received the packages.
"We are not aware of any delivered elsewhere in the country.
"We want to reassure the public we are doing everything in our power to get to the bottom of this matter."