The AVID investigation, which was previously classified, prompted the Netherlands to remove two members of the Iranian embassy staff a year ago after Denmark blamed Tehran for plotting to attack three Iranians alleged to be members of the Arab Struggle Movement for the Liberation of Ahvaz (ASMLA).
In a statement on Tuesday, the Dutch foreign ministry finally said it had "strong indications that Iran was involved in the assassinations" and that it kept the evidence secret "in the interests of facilitating this common European Union action" against Iran.
Two years later, the Iranian activist Ahmad Molla Nissi, 52, who founded an Arab nationalist group seeking an independent state inside Iran, was also shot in the head as he walked through The Hague.
"Europeans, including Denmark, Holland, and France, harbour MEK - who killed 12,000 Iranians and abetted Saddam's crimes against Iraqi Kurds - as well as other terrorists staging murder of innocent Iranians from Europe", Iranian foreign minister Javad Zarif said on Twitter.
France previous year imposed sanctions on two suspected Iranian agents and others from Iran's intelligence and security ministry.
On Tuesday, the Netherlands said it had "strong indications" that Iran was behind the assassinations of two Dutch nationals of Iranian origin, in 2015 and in 2017.
"The EU and the Netherlands take strong action against Iranian unlawful interference in Europe".
"Accusing Iran won't absolve Europe of responsibility for harboring terrorists", he wrote.
"Iran is expected to cooperate fully in removing the present concerns and, where necessary, aiding in criminal investigations".
"If such cooperation is not forthcoming, further sanctions can not be ruled out", it added.
Dutch Minister of Foreign Affairs Stef Blok said Iran also attempted an assassination in Denmark and a bomb attack in France.
The sanctions are the first the European Union has imposed on Iran since the nuclear accord three years ago.
"We will continue to support the nuclear agreement provided that Iran complies with it". The agency that is monitoring this says that they are. Tehran denied the accusations as "hostile" and said they were in line with the "enemies' plots" to undermine Iranian-European relations.
Tehran has blamed ASMLA for the September 28, 2018 attack on a military parade in Ahvaz, which left 30 people dead and 70 injured. ASMLA, pushing for a separate Arab state within Iran, is classified as a terrorist organization by Tehran.
"Three decades of experience show that the mullahs only understand the language of firmness", it said in a statement.
The Netherlands previously expelled two Iranian embassy workers in connection with the deaths.
Tehran at the time protested the expulsion as an "unfriendly and destructive move" and threatened to retaliate.