[Supposedly]

Apart from Omega's and FINA's statements that Cavic touched first, but Phelps was the first to trigger the mechanism with enough force, you can find the photos here

https://static01.nyt.com/images/blogs/olympics/phelpscavic53...

https://media2.s-nbcnews.com/j/msnbc/Components/Photo/August...

https://bloximages.newyork1.vip.townnews.com/herald-dispatch...

It is a shame to lose a gold medal this way. Sometimes, however, there is nothing you can do.