German prosecutors are investigating three people over a deadly explosion at a waste treatment plant in July.
Authorities in Cologne confirmed that a preliminary investigation is now underway against three employees at the Chempark industrial area.
The suspects are facing allegations of negligent homicide and negligently causing an explosion, according to the city's public prosecutor's office.
Mobile phones and documents were seized during searches at the homes of three individuals on Tuesday.
The employees are suspected of violating their duties of care in connection with the storage and handling of the waste at the industrial park.
Seven people were killed and a further 31 people were injured in the deadly blast in Leverkusen on 27 July.
A provisional report in August found that the explosion was likely caused by a chemical reaction due to high pressure within one of the facility's tanks.
The public prosecutor's office and the Cologne police are investigating further.