I think someone poisened my cat.?

I am so upset I had to have my cat put down last week by one of my neighbors(don't worry it was done humanely) he had a seizure that lasted over 2 hours and showed no sign of ending, also his back was soaked with a citrus smelling type substance. Could anyone please tell me what could have been poured on my poor deceased cat Ashes. I miss him terribly and am trying to get as many answers as I can.