Barcelona attack witness saw driver 'knocking people over at high speed'

A tourist who witnessed the terror attack in Barcelona, Spain, today that killed 13 people and injured at least 100, told ABC News he saw a driver "knocking people over at high speed."

Brendon Sissing told ABC News he heard screams of people to his left, and when he looked up, he saw a white van driving at what he estimated to be 50 to 60 miles per hour down La Rambla, a popular street with tourists and locals alike.

The van knocked people over, and some people were under the van, but the driver "just kept going," Sissing said.

The driver stopped opposite to where Sissing was standing, he said, and then kept going.

Sissing ran into a shop, he said. When he left, he heard about five gunshots, so he ran into a second building where there were construction workers.

One witness told a Spanish television station that after the suspect ran over people in his van for nearly 1,000 feet, he fled into a nearby Turkish restaurant.

Police said two people are detained in connection with the attack, for which ISIS has claimed responsibility.

U.S. Secretary of State Rex Tillerson offered condolences for the "loss of life and injuries" of "so many innocent people, yet again."

To terrorists around the world, Tillerson said, we are "resolved to find you and bring you to justice."

Tillerson added that the U.S. stands ready to assist authorities in Barcelona.