BRIDGWATER, N.J. >> The United States swiftly condemned today’s deadly terrorist attack in Barcelona and offered assistance to authorities in Spain.

President Donald Trump, who remained out of public view for a second straight day, denounced the attack in a statement on Twitter. “The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help. Be tough & strong, we love you!” the president said.

Trump is on a working vacation at his private golf club in New Jersey. He was being briefed on developments in Spain by his chief of staff, John Kelly, the White House said.

First lady Melania Trump tweeted her “thoughts and prayers” to Barcelona before the president condemned the attack.

At least 13 people were killed and dozens more were wounded after a van was driven onto a sidewalk and down a pedestrian zone in Barcelona’s historic Las Ramblas district.

Secretary of State Rex Tillerson offered condolences to victims of the attack during an appearance before journalists at the State Department. He said the incident “has the hallmarks of what appears to be yet another terrorist attack.”

Tillerson said U.S. diplomats in Spain are assisting Americans there, and he asked those who are safe to notify friends and families. Tillerson said the United States will never relent in tracking down terrorist suspects and holding them to account for their actions.

“Terrorists around the world should know that the United States and our allies are resolved to find you and bring you to justice,” Tillerson said.

Spanish authorities tweeted that a man was under arrest and being treated as a terrorist.