Six cats all living in the same Northampton street have been found dead during the last month.
The cats were all being temporarily homed by an animal charity in Obelisk Rise in Northampton but were not caged.
Two cats were found dead on May 10, three between May 17 and 19 and one on June 8.
A spokesman for Northamptonshire Police said: “A post mortem test on one of the cats revealed that it had died of kidney failure due to ingesting anti-freeze. The other five cats all displayed the same symptoms and it is believed that they must have come into contact with anti-freeze too.
“It is not known how the cats came to be poisoned in this way, and police are investigating whether there are any suspicious circumstances.”
Witnesses or anyone with information about this incident is asked to contact police on 101 Crimestoppers anonymously on 0800 555111