REVEALED: The Michigan teenager who died mysteriously on Royal Caribbean's Harmony of the Seas as FBI probes death

A devastated Michigan family has paid tribute to their 'brilliant and witty' son after he fell to his death from a cruise ship balcony in an 'unthinkable' accident.

Pierson 'Piers' Davis, 12, was with his mom, dad, and two sisters aboard the Royal Caribbean ship Harmony of the Seas on the final evening of its seven-day voyage to the Western Caribbean when the tragedy struck on September 7.

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

The investigation into his death has been taken over by the FBI, and Piers' family gathered on Saturday near their home in Coloma for a funeral on what would have been their son's 13th birthday.

'The thing every parent hopes and prays does not happen to them. It happened to me,' his mother Audra Davis wrote ahead of his funeral. 'It happened to us.'

'I don't have many words right now,' the 47-year-old added. '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.'

Piers was the 'cherished middle child' of his parents and was on board with sisters Amelia and Zuzana for what fellow passengers thought was a birthday celebration.

Becky Potter and husband Gavin befriended the family while on board and took a picture of the families dining together.

'We walked away talking about your precious family and your dedication to your kids!' she told Piers' mother.

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

Dad Justin works as a screenwriter with credits including the reality TV shows Pretty Wicked, Amazing Race and Date My House to his name.

His wife of 19 years is a manager in the compensation operations department at Whirlpool in Benton Harbor where colleagues have started a Gofundme appeal to help the family.

As of Wednesday morning it had smashed its $7,000 target with nearly $30,000 donated by well-wishers.

'Our coworker and friend, Audra Davis has suffered an unimaginable loss,' appeal organizer Sara McLeod wrote on the page.

'Her sweet son, Pierson (Piers) passed away this past weekend in a tragic accident.

'Audra has always been a positive inspiration to our team, and we are hoping to help in any way we can.'

The French-built ship was the world's largest cruise liner when it was launched in 2016, with rooms for more than 7,000 passengers across its 18 decks.

Geared towards family-bookings it boasts 'endless opportunities' for children to 'make friends and engage in fun, educational, supervised activities with other cruisers in their age group'.

The boy had taken part in the cruise's teen-club, and many passengers said their children had got to know the boy before his death.

'The conversations I have had to have with my son really tripped me up,' wrote Shannon Elizabeth from Belfair in Washington.

'My son and him actually got really close on the cruise, and he saw too much of what happened.'

In their obituary Piers' family described him as 'wise beyond his years', with a 'natural confidence and social ease'.

'Despite his age, he possessed a mature sense of gratitude and a heart full of love, never hesitating to express it,' they 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.'

Witnesses said Piers had been playing with a group of friends he had made on board shortly before the accident, with some suggesting chairs had been pushed up against the balcony railing.

'My family and I saw the incident unfold right before us as we were looking down into Central Park from the pool deck,' wrote Christa Schoolfield. 'We did not see whether he jumped or was playing around.'

'It had rained that afternoon and the balconies were damp,' Potter added.

'It would be difficult to fall if you stand on the ground, but if on the chairs or wet table top anything is possible.'

Passengers on the Harmony had already been unnerved by a series of 'alpha alpha' emergency alerts during the cruise by the time the final one signaled the 12-year-old's fatal fall.

Royal Caribbean staff attempted to resuscitate the youngster, but he died before the ship docked in Texas.

Border patrol agents confirmed they were initially involved in the investigation before handing over to the FBI.

'On September 7 CBP was notified of the death of a passenger on a cruise ship while at sea,' a spokesman said.

'Upon arrival to the Galveston Seaport the following day, CBP officers were asked to briefly assist cruise employees on the ship until other law enforcement arrived to investigate the incident.'

The FBI refused to share details of the investigation other than to say that it 'remains ongoing'.

'An FBI Maritime Liaison Agent, based out of the Texas City Resident Agency, is coordinating with the US Coast Guard, Customs and Border Protection, and the cruise line regarding this incident,' a spokesman said.

Piers' mother said she had been sustained by the hundreds of people who have reached out to her since her son's death.

'Thank you, I love you. Even if I don't know you personally, I love you,' she wrote on Facebook.

'The kindness of strangers during this time has been supernatural.'

Royal Caribbean has refused to comment.