PHILADELPHIA - Singaporean Thia Shan Zhi, 21, was on a bus to Boston when traffic came to a standstill not far from the city centre.

Sirens blared as surrounding vehicles made way for ambulances and police cars rushing down the adjacent roads.

Mr Thia, a volunteer at the Boston Marathon, had no idea then that two bombs had exploded near the finish line, killing two people and injuring more than a hundred.

He had been handing out water and sports drinks to runners in a checkpoint at Natwick, midway through the 42-km race. Having finished his duties, he and the other volunteers "wanted to come out and enjoy the occasion", the Tufts University student told The Straits Times.

However, when Mr Thia reached the Prudential Center - an iconic building on the same street as the crossing line - it was not a triumphant celebration of athletic feats that greeted him but "thousands of people running away," many of them crying.

Even then, it was not until he checked his Twitter and text messages that he realised what had happened.

"There was a lot of confusion on the bus back - our main priority was to find out if our fellow students were safe," he said.

Although his friends have been accounted for, the city struggles to piece together what happened, and why.

Summing up his experience, Mr Thia said: "I'm still trying to make sense of all this."