Prosecutors on Wednesday charged a Turkish man and his American fiancee with conspiring to attack a U.S. military base last year.

Osman Petmezci, 25, and his fiancee Astrid Eyzaguirre, 23, who were arrested in early September, were charged with "plotting to carry out an explosion...at the U.S. military base in Heidelberg," city prosecutors said in a statement.

Investigators found nearly a pound of gunpowder and six pipes at the couple's apartment in Walldorf, south of Heidelberg, as well as other chemicals that they believed could be used to make 250 pounds worth of explosives, prosecutors said.

The couple deny they planned any attack, according to the statement.