This reward is in addition to the $1,200 being offered by the Suffolk County SPCA, which is leading the investigation, bringing the total to $3,700.
Authorities say the black-and-gray kittens were found Tuesday morning behind Riverhead Brake Service on East Main Street.
The SPCA says the kittens likely were dumped there between 5 p.m. Monday and 6:30 a.m. Tuesday.
SPCA head Roy Gross said the kittens were about 3 weeks old. He says they were about the size of a hand.
Anyone with information is asked to call the SPCA at 631-382-7722. All calls will be kept confidential.