The official died two months after overseeing matches at the Rio 2016 Paralympic Games, after suffering a brain aneurysm aged 44.

Apol also officiated in beach volleyball at the Rio 2016 Olympics, becoming the first sitting volleyball referee to be given this honour.

He is credited with being a key contributor to ParaVolley Beach and first became involved in volleyball in 1987.

In 2006, he became a volunteer coach of the American sitting volleyball teams, while his officiating career started as a line judge at the 2010 World Championships.

He passed the international referee course in 2011 and was appointed to serve at the 2014 World Championships before being called up for the Paralympics.

"Despite all his successes Dan always remained grounded," World ParaVolley said.

"He was always professional, was highly respected and a role model. 

"It was typical of his approach to life that as an organ and tissue donor he was able to better the lives of more than 50 people.

"His life and World ParaVolley career ended way too soon. 

"He contributed a great deal to the sport and would undoubtedly have gone on to contribute for many years to come."