German police have removed boxes of ice pack gels used to make explosives from properties linked to four men arrested on Thursday night over a Hamas plot to attack Jewish institutions in Europe.

Officer were seen carrying blue boxes which were said to contain data storage devices, ice pack gels and other evidence.

The ice packs were seized because the gel in them contains ammonium nitrate, which can be used to make improvised explosives, German tabloid Bild reported.

All four suspects are members of Hamas with links to its military wing, German prosecutors say. Three of them were arrested in Berlin and the fourth was held in the Netherlands.

The men are accused of trying to bring a cache of weapons from an undisclosed location in Europe to Germany to attack Jewish institutions.

Hamas, which is proscribed as a terrorist group in the EU, the United States and Britain, controls the Gaza Strip and launched the unprecedented October 7 massacre in southern Israel.

Police in ski masks used sniffer dogs to carry out searches of four apartments and a restaurant linked to the plot, which Israel has suggested was part of a wider cross-border terrorist operation.

Chemicals typically found in gel ice packs were reportedly used in the 1995 Oklahoma City bombing which killed 168 people, the 2011 Delhi bombings which killed 17, and in a car bomb deployed by Norwegian terrorist Anders Breivik.

According to the UK government’s import guidelines, ammonium nitrate is a “highly dangerous chemical compound” which can also be found in certain fertilisers.

The Hamas plot was being planned many months before the October 7 massacre in southern Israel, with German authorities launching their investigation over the summer.

A further three people with alleged links to a Hamas terror plot have been arrested in Denmark, with the country’s prime minister describing the alleged offences as being “as serious as it gets”.

Israeli officials have suggested that all seven suspects were part of a cross-border terrorist plot against Jews in Europe.

Europe has been on high alert for Islamist or Hamas-inspired terror attacks since the October 7 onslaught, which triggered the biggest ever war between Hamas in Gaza and Israel.