Berlin CNN  — 

Authorities are investigating the deaths of 21 people at a company in Germany after an employee was allegedly caught poisoning a colleague’s lunch.

A 56-year-old man has been charged with attempted murder at a company in the northwestern town of Schloss Holte-Stukenbrock, said the police in nearby Bielefeld, where the man lives. Police did not name the suspect.

He was seen on a security camera opening a co-worker’s lunchbox and putting a substance on the sandwich inside, the police said in a statement.

A small bottle of “powdery substance” was found in the suspect’s bag after he was taken into custody, police said.

The sandwich’s owner had raised the alarm earlier after discovering an unknown substance smeared on his lunch. He informed his company’s management, who in turn notified police.

Testing by the regional criminal office of North Rhine-Westphalia indicated the substance on the bread was toxic lead acetate, and there was enough to cause severe organ damage, authorities said.

The accused was brought before a judge on May 17, who issued an arrest warrant for attempted murder.

Fire brigade specialists found mercury, lead and cadmium in the suspect’s apartment in Bielefeld. Police said the man “has long tried to produce toxic substances, including heavy metal compounds,” based on substances found in his home.

Authorities broadened the investigation after two other cases of illness at the company in recent years were discovered. Officials are also now re-examining all deaths connected to the company since 2000 – and in particular, 21 former employees who died before retirement – to see if heavy metal poisoning played a part.

“There were a strikingly high number of heart attacks and cancers among the death cases in the company,” the police statement said.

Police said they will question relatives and former physicians, and review medical records. It’s possible the bodies might have to be exhumed.

Police said they know of no possible motive.

CNN’s Sophie Rebmann reported from Berlin, and Lauren Said-Moorhouse wrote in London.