Police discovered "explosive and further items in their car" as well as a note "indicating a planned attack on the branch of an international company," Jeannette Balmer, a spokeswoman for the federal prosecutors office, said.

All of those arrested remain in detention, she said.

"IBM has been notified by Swiss police authorities about an investigation and is co-operating accordingly," Chris Sciacca, an IBM spokesman, said.

He declined to answer further questions, citing the police investigation.