I don't use medicated feed anymore because I have seen that it doesn't really help prevent cocci. I haven't had any cases when I used it, or for the last few years hatching new chicks. I would recommend just keeping a bottle of Corid on the shelf if chicks develop symptoms. I recommend...
I'm sorry for your loss, but the use of medicated feed does not cause a thiamine deficiency. Amprollium acts by blocking thiamine uptake in the coccidia oosysts, but it does not cause thiamine deficiency in the chicks.
If the chickens were both inside and outside, and couldn't get botulism or poisoned as Nambroth suggested, I would suspect the feed. I have seen several cases people on BYC who have lost many in their flocks from something wrong with their feed. Aflatoxin posioning, aspergillosis due to mold...