Career
He competed for Great Britain at the 2004 and 2008 Summer Olympics and was chosen as the sole male cross-country rider for the British team for the London Games in 2012. Killeen had been interested in cycling from a young age, but a trip to the Malvern Classic introduced him to cross-country cycling when he was about thirteen. He had previously been interested in motocross.
Competing in the Games themselves for the British team, he was held up by a crash early on in the race and ended up finishing in fifth place, outside of the medal spots.
He returned to the English team at the 2006 Commonwealth Games in Melbourne, Australia. Having placed third at the previous Games, he went two places better, taking the gold medal for his country.
In 2007, he suffered from chronic fatigue syndrome and could not compete for an entire season. He competed once more for the British team at the 2008 Summer Olympics in Beijing, and was considered one of the favourites for the gold medal.
Only 150 metres (490 ft) into the race, his handlebars clipped a race marker, flipping him over the bars and off the bike.
This left him in last place chasing the rest of the pack, but he managed to move up to seventh place by the finish. He considered a switch to road racing to take part at the Games. He was the only athlete competing for Britain in the men"s cross country.
As part of his training, he conducted practice runs at the Olympic track near Southend.
He competed at the 2014 Commonwealth Games, finishing in 6th place.