"We are unfortunately talking about several people dead and several injured," said Wolfgang Wierich, spokesman for Düsseldorf police.
The victims were found in a house at Schwalmtal near Düsseldorf, police said. Officials declined to comment on reports that the suspected shooter was a father who had been in the middle of a family break-up.
"The person responsible has been arrested," the spokesman said.
The Rheinische Post reported on its website that four people had died and at least one person was seriously injured after a divorced couple met in their home. They had come with their lawyers and real estate agents to determine how to proceed with the sale of their property.
The shooting reportedly occurred after a dispute broke out and the situation escalated.