Authorities opened an investigation into 21 deaths at a company in northwestern Germany, after an employee was caught on camera poisoning a colleague's lunch, police said.

Police said they had arrested a 56-year-old man suspected of attempting to kill a colleague at the company in the town of Schloss Holte-Stukenbrock.

They did not identify the firm, the suspect or the alleged victim.

The victim had noticed something suspicious in his lunch and notified his bosses and the authorities, police said.

Video footage revealed the suspect spreading a powdery substance on the victim's food.

Tests showed it was a poison that could have caused severe organ damage.

During a search of the suspect's apartment, police found toxic chemicals such as mercury, lead, and cadmium, they said.

Further investigation uncovered 21 cases, dating back to 2000, of employees at the same company who had died of heart attacks or cancer shortly before retiring.

Experts concluded that heavy metal poisoning could have been the cause of the illnesses that led to those deaths.

The suspect's motive was not clear because he remained silent about the allegations, the police said.