aws s3 calls failed with access denied when app is deployed in beanstalk

I have a small nodejs application that uploads files to s3. All is up and running and tested. The problems arose when I deployed my application in Amazon Beanstalk. I managed to do that but my s3 calls failed with access denied. I experimented with different bucket policies with no result. To be honest I am not sure what are the user credentials and even who is the user for which access is denied.