The pair had allegedly researched various forms of explosives before deciding on the deadly poison. The couple, who have four children, bought a hamster to test the strength of the poison, according to prosecutors.

Sief Allah H., a former street vendor and labourer in Tunisia, in 2015 married Yasmin H., an unemployed doctor’s assistant and mother of seven children from four different fathers.

The husband had been in contact with radical Islamists and tried twice in 2017 to travel to Syria via Turkey. His wife helped him with flight and hotel bookings, but both trips failed.

Sief Allah H. also volunteered to help Islamic State terrorists in their propaganda work, and did so in early 2018 by publishing material of the jihadist group online, said prosecutors.

Later, the couple decided to prepare an attack in Germany itself.

“Very concrete preparations had been made for an act with a … biological bomb, which is a first for Germany,” the Federal Prosecutor’s Office said.

If convicted of the charges of serious violence endangering the country, the defendants could each face up to 15 years in jail.

Two other co-conspirators were also arrested in August last year in Tunisia in connection with the case. According to German newspaper Die Welt, it was the U.S. CIA that tipped off German intelligence about the migrant’s plan which resulted in the raid and arrests.

The trial is expected to last until the end of August.

AFP contributed to this report