Counter-terrorism authorities in Europe warned of the risk that refugees could be targeted by extremist recruiters weeks before the Berlin Christmas market carnage.
The threat was highlighted in a report published by Europol, the EU's law enforcement agency, at the start of this month.
It also cited unconfirmed reports that German authorities were aware of hundreds of attempts by jihadists to recruit refugees.
And the study, which examined the tactics deployed by Islamic State (IS), also said Germany may be considered a target of the group "because of the large influx of Syrian refugees in the country, belying the promise of the Caliphate as an Islamic utopia".
Germany has taken in hundreds of thousands of asylum seekers since the international migration crisis started last year.
The country was rocked by a suspected terrorist attack on Monday when a truck ploughed into a crowded Christmas market in Berlin, killing 12 people and injuring dozens more.
Unconfirmed reports by German media have indicated that security sources believe the man behind the wheel was an asylum seeker from Pakistan or Afghanistan, thought to have come to Germany in February.
The Europol report said: "Elements of the (Sunni Muslim) Syrian refugee diaspora may become vulnerable to radicalisation once in Europe and may be specifically targeted by Islamic extremist recruiters."
There is no firm evidence that terrorist travellers "systematically use the flow of refugees to enter Europe unnoticed" but it is indisputable that some have entered the EU posing as refugees, the paper added.
It said: "According to unconfirmed information, German authorities were aware of around 300 recorded attempts made by jihadists to recruit refugees who were trying to enter Europe by April 2016."
Germany, the UK, France, Belgium, and the Netherlands were identified in the assessment as being "high on the target list" for IS aggression.
The Berlin crash came less than a month after Americans were warned of the heightened risk of terrorist attacks throughout Europe, particularly during the holiday season.
A travel alert issued by the US State Department specifically urged caution at holiday festivals, events and outdoor markets, saying that "credible information" indicates that IS - also known as Isil or Daesh - al Qaida, and affiliates continue to plan terrorist attacks in Europe.
Both IS and al Qaida have previously urged followers to use vehicles to carry out attacks, and the Berlin incident had echoes of the Nice atrocity in July.
For more than two years the official threat level for international terrorism in Britain has stood at severe, meaning an attack is "highly likely".
Earlier this month, MI6 chief Alex Younger used his first public speech to warn that the "murderously efficient" IS was plotting violent attacks against the UK.
Authorities are dealing with around 550 "live" cases at any one time, while intelligence and security services have disrupted 12 terrorist plots over the last three years.