(CNN) -- Police in Germany are investigating after sandwiches left on the doorstep of a company labeled as a "present" turned out to be spiked with rat or mouse poison.
About 25 workers at the company in the town of Steinfeld, Lower Saxony, helped themselves to the filled bread rolls Tuesday after they were left in two boxes that said they were to be eaten, police in nearby Cloppenburg said.
Later that afternoon, an alarm was raised when a strange substance was noticed atop the bread rolls, the police statement said.
The discovery prompted 11 people to admit themselves to local hospitals, but no one has shown any symptoms of illness as a result of eating the bread rolls, police said.
The substance was flown to a Berlin hospital for tests that confirmed it was rat or mouse poison. The strength of the poison is not yet known, police said.
Police said Thursday that they have spoken to all 25 people affected, but no new leads have been confirmed.
Officers believe that an unknown person dropped off the boxes of sandwiches, complete with paper plates and napkins, before 6:30 a.m. Tuesday.
Witnesses said the bread seemed fresh, so the police are eager to find out who might have bought a large number of rolls in the area on Monday evening or Tuesday morning. They are also looking into recent purchases of wheat-based rodent poison in the area.
The company involved, Mueller-Technik, declined to comment on the incident Thursday. It makes car parts and employs 210 people, according to its website.
Steinfeld is a town of about 9,500 inhabitants.