FBI probe death of 12-year-old boy who died mysteriously on cruise ship

The FBI has opened an investigation into the death of a 12-year-old boy on a cruise ship.

Pierson 'Piers' Davis had been on vacation with his parents and two sisters on the Royal Caribbean cruise ship Harmony of the Seas.

The vessel had been on the final evening of its seven day voyage to the Caribbean on September 7.

Around five hours before the ship was due to dock in Galveston, Texas, Piers fell from his family's 13th floor cabin balcony five stories into the Central Park plaza five stories below.

Royal Caribbean staff tried to resuscitate the Pierson, but he sadly died before the ship was able to dock.

The family have paid tribute to Pierson who was on vacation with his family when the tragedy occurred. (Family Handout)

Now, the FBI has taken over an investigation into Piers' death as his family remembered him on what would have been his 13th birthday.

Piers' mom, Audra Davis, paid tribute to her son ahead of his funeral, writing: "The thing every parent hopes and prays does not happen to them. It happened to me. It happened to us."

She added: "I don't have many words right now. My family and I were on a cruise vacation the first week of September and the day before we were to arrive at port the unthinkable happened.

"My son tragically died. He was 12 and would have turned 13 later this month.

"To all those who have reached out to me and my family to offer your support, love, prayers and good intentions, thank you. It means everything to us."

While on board, the family befriended Becky Potter and her husband Gavin, with the couple taking some pictures of the families having dinner together.

"We walked away talking about your precious family and your dedication to your kids!" she said to Piers' mom.

"My heart couldn't break any more for such an amazing family!"

Pierson 'Piers' Davis fell from a balcony on a cruise ship (David Sacks / Getty Images)

An obituary by Piers' family described him as having a 'natural confidence and social ease', as well as being 'wise beyond his years'.

"Despite his age, he possessed a mature sense of gratitude and a heart full of love, never hesitating to express it," the family wrote.

"His mind was sharp, constantly improving his skills and knowledge, often in Pokémon, other video games and any new mystery he could strum up.

"As an avid learner, he was fascinated by Mars and the mysteries of space, wanting to delve into the universe as a means to explore his philosophical thinking.

"Recently, Piers discovered a love for golf, but his true peace was found in the woods, where he could reflect and immerse in nature, this was a place he loved to sit and ponder world secrets, like how to make it a better place."