The letter bombs have been sent to, among others, the Ukrainian embassy in Madrid, to an air base and to the Spanish Prime Minister's private residence.

Spain's support for Ukraine after Russia's invasion is a possible motive behind the attacks, which are now being investigated as terrorist crimes.

Terror expert Hans Brun thinks so.

- But it is important not to lock yourself into a theory.

It could be anything from someone who is dissatisfied with these institutions to more advanced planning linked to the war, where they want to disrupt Spanish support for Ukraine's fight against Russia.

- The choice of targets is interesting, they have received quite a few shipments for important instructions.

It indicates a certain degree of planning and ability to gather enough materials.

If more shipments like this appear, it will be interesting to follow what type of targets are attacked and, if so, where, says Hans Brun.

The goal is to alarm the public

Letter bombs have been used since at least the 1970s by, among other things, terrorists.

The most famous example is perhaps the so-called Unabomber Theodore John "Ted" Kaczynski, who spread terror in the United States with letter bombs he sent to airlines, universities and private individuals.

Between 1978 and 1995, Kaczynski sent off a total of 16 bombs in what he called a protest against civilization's encroachment on the wilderness around him.

Kaczynski's bombs killed 3 people and injured 23.

- Otherwise, the purpose of letter bombs is above all to make it public and for the public to be concerned, says Hans Brun.

Must check every letter

That the letter bombs in Spain have only been discovered when the letters have arrived at embassies, residences and companies is not surprising, says Hans Brun.

Going through each envelope manually is pretty much the only way to detect potentially dangerous shipments.

- The post office around the world handles so many letters, which means that you have limited opportunities to detect this type of shipment.

The safest and easiest place to go through the letters is at the recipient's place, such as an embassy, ​​a parliament or a company.

That is why the resources are put there.

In the clip above, hear Hans Brun answer three questions about the letter bombs in Spain.