The explosion of a letter bomb at the headquarters of the ING bank in Amsterdam

A letter bomb exploded today at the headquarters of the ING bank in Amsterdam, but there were no casualties, the police said.

It happened a day after letters containing explosives were sent to two other companies in the same city.

Only one employee of the bank, who was suffocated by smoke, was taken care of by the rescue services, Pollici said on Twitter.

The second bomb letter was found this morning in the premises of the American company Unisis, in Lusden in the center of the Netherlands, and that bomb was dismantled by the special units of the Ministry of Defense.

Yesterday, two letter bombs exploded, also without casualties, at the premises of the ABN AMRO bank in Amsterdam and the Japanese Ricoh group in Kerkrade.

"It is possible that these incidents are connected," the Dutch public broadcaster NOS reported, citing police sources.

The sender of those letters threatened other explosive attacks unless an unspecified amount of bitcoins was paid.

Police strongly believe Wednesday's blasts are linked to a series of letter bombs, at least seven of them, sent to various companies and hotels in December and January.