if a customer disputes an EFT, what could be considered a reasonable investigation, can we rely on the merchant statement?

Have you read 1005.11(c)(4) and associated commentary?

I think it would depend on the merchant statement - I've seen a few that aren't actually grounds for denying the claim. Many times, merchants provide information that actually proves that the client did NOT do the transactions. A great example of that is four transactions on Uber within ten minutes, two of which were for rides in CA , two for rides in NY. If we got a merchant statement that actually showed customer did transaction, we would normally call the customer to discuss before removing provisional credit. Obviously not mandatory, but goes a long way toward helping the customer understand.

thanks will look into it, i was thinking more that the vendor told us that the transaction was done using the customer's card, but the customer is telling us that she did not authorize it

The EFTA says the burden is on the bank to prove your customer did the EFT, authorized it or benefitted from it. If you review your file and have questions, doubt the "reasonableness" of your investigation. If you answer the questions and your evidence shows it was your consumer or someone authorized, you did a good job.

What you cannot do is send them back to the merchant or make an assumption that "you said you always had your card and your card was used, therefore you did this." You have to ask who, what, when, where and why. We had a case where a purse was not secured at the customers work. Someone was getting her card regularly, using it and replacing it. Was it the customer's card - Yes. Did they feel they always had it, Yes. Was it them on film, No.

To add to Andy's statement - counterfeit cards can happen, too. Just because a card was used actually proves very little.

Thanks! i also think that if the vendor says that they receive the request and made the payment it is not enough for us to say it was authorized, anyone nowadays can get fraudulent access to cards or other information. i think we should investigate more and not rely so much on the vendor right?

Right, the vendor obviously received the information needed to process a payment, otherwise there wouldn't be a dispute.