Extremists from across Europe joined Daesh in droves in 2014, when the Takfiri terror group launched its campaign of death and destruction in Iraq and Syria.
According to the commission, at least 5,500 foreign terrorist fighters left the continent to travel to Iraq and Syria, said Julian King, the European Commission for security.
Of those two-thirds were men, and a quarter women.
“We think at least 1,400 were killed, died,” King said, adding that around 1,600 have since returned.
“That leaves 2,500 unaccounted for, we don’t know where they are,” he added.
Many warned that those returning home will be a security challenge for years to come.
They include half of the estimated 850 people who left the UK to join Daesh and also 400 of the 3,417 militants from Russia; 760 of the 3,244 from Saudi Arabia; 800 of the 2,926 from Tunisia; and 271 of the 1,910 from France.
Back in January, France said it was considering the repatriation of 130 men and women to be tried under the French judicial system.
German Daesh woman saw 5-year-old girl die of thirstA German woman is charged in Munich with letting a five-year-old Iraqi die of thirst while she was a member of Daesh in Mosul.
In Germany, a woman was charged with war crimes late last year for allegedly letting a five-year-old girl die of thirst while serving as a member of Daesh in Iraq.
German prosecutors charged the 27-year-old woman, only identified as Jennifer W, with war crimes, murder and weapons offenses earlier this month at a Munich court.
Police said that the woman had first left Germany in August 2014 and traveled via Turkey and Syria to Iraq where she joined the terrorist group the following month
In January 2016, she allegedly visited the German embassy in the Turkish capital, Ankara, and applied for new identity documents. She was arrested by Turkish security services after leaving the embassy and deported to Germany a few days later.
Europol’s annual EU Terrorism Situation and Trend Report (TE-SAT) warned last year that a particularly strong security threat is posed by terrorists, who have received military training in the use of weapons and explosives, or have gained combat experience during their stay in Iraq and Syria.