A Richmond businessman has told how he searched for his young son when he crossed the finish line of the Boston marathon – after the explosions had gone off yards from him.

Simon Jones was about to cross the 26-mile mark when he heard the first of two blasts, which have claimed three lives and left more than 170 injured.

He said: “There was a very loud crack like a firing pistol. It’s not a boom, it’s a crack. Then there was a large cloud of smoke and the barrier started falling.”

He carried on running through what has been described as America’s worst terror attack since 9/11 to find his seven-year-old son, who he had arranged to meet at the end of the race on Monday, April 16.

The 47-year-old said: “You turn the last corner and you see the 22.6 mile sign in front of you. I hear the loud crack and saw some guys falling out of shock.

“I kept going and then the second explosion was next to me. I was calm. I was fine. I was just concerned about Jack.”

He ran through, without getting any food or water, to the family meeting point to find his son, who was not there.

Mr Jones, who regularly runs in Richmond Park, received a text from a friend he studied with at nearby Harvard University, to tell him they were safe back at the hotel after somebody warned him not to take Jack to the finish line.

He said: “It was about half an hour later that I saw them. I was just relieved. You have so much adrenaline in your system keeping you going. I was just relieved that they were both OK.”

The following day he took his son to a duck centre in Cambridge, about six miles from Boston, to get away from the media storm and disorder in the city.

Police investigators said the bombs, which exploded within seconds and about 50m of each other at about 2.50pm on Monday, were made from kitchen pressure cookers packed with nails.

Mr Jones said: “People were clearly concerned, there were some tears, but there was not mass hysteria and people were very kind to each other.

“There was a rush of people shouting and running, but I thought generally people were very calm.”

The turnaround director, who finished the race in four hours and three minutes, was running his fifth marathon and had not previously competed in Boston. He still plans to take on the Chicago marathon in October.

He said: “I was really fortunate, but it’s really tragic for the people who were hurt and you wonder how you survive that and I just feel very sorry for them.”