Extremists have plotted to use Covid-19 as a biological weapon to carry out attacks during the pandemic.
Islamist and far-right terrorists have discussed using the virus against targets including politicians, police and minority groups.
The findings are part of a 113-page Europol report that says terrorists sought to take advantage of the pandemic by sowing division and hatred.
It is the latest in a series of warnings that detail how the social and economic fallout from the pandemic could be fertile ground for extremists.
The report says lockdowns have forced terrorists to rely on low-tech methods, such as knife attacks, vehicle ramming and homemade explosives.
Amid the chaos caused by the pandemic, discussions in some online circles turned to using biological weapons.
Suggestions included handing out poisoned masks or deliberately spreading Covid-19, for example by shipping contaminated products.
“Terrorist propaganda and online chatter suggested possible ways of weaponising the virus,” the report said.
“Close contact, airborne and fomite transmissions [via surfaces] were suggested as sources of contamination targeting minorities, politicians, police officers and medical staff.
“Right-wing extremists further suggested attacks on critical infrastructure, governmental facilities and the use of cyanide to contaminate drinking products.”
The idea of using the virus as a bioweapon also surfaced in Islamic extremist propaganda, Europol said.
But terrorists and extremists often lack the scientific knowledge to be able to use the virus as a weapon, and no such attacks have yet taken place.
Terrorist attacks in Europe in 2020 included the assassination of a French school teacher last October and a shooting rampage in Vienna in November.
In Germany, a far-right extremist killed 10 people after opening fire at two shisha lounges before the pandemic took hold in Europe.
Once it did, schools and businesses moved online in a trend that experts fear could make young people particularly vulnerable to radicalisation.
Europol said that some of the lone actors who carried out attacks may have been radicalised online.
“The online domain plays a crucial role in enabling the spread of terrorist and extremist propaganda,” said Europol’s executive director Catherine De Bolle.
“In a world which has become considerably more digital, targeting the propagation of hatred and violent ideologies spread online is an imperative.”
Ylva Johansson, the EU’s commissioner for home affairs, said the terror threat had grown especially among far-right extremists during the pandemic.
The far-right has been linked to anti-lockdown protests and Covid-19 conspiracy theories, including in Germany.
The latest report “illustrates that in the year of the Covid pandemic, the risk of online radicalisation has increased,” Ms Johansson said.
“We are committed to tackling this growing threat.”