Secretary of Homeland Security Kirstjen Nielsen vowed her agency was helping to secure New York City after an attempted bombing there Monday morning.

"The Department of Homeland Security is taking appropriate action to protect our people and our country in the wake of today's attempted terrorist attack in New York City," she said. "We will continue to assist New York authorities with the response and investigation and we urge the public to remain vigilant and report any suspicious activity."

"The enemy we face is persistent and adaptive," she added. "But they should know this: Americans will not be coerced by terrorism, and we will not allow it to become the new normal. We will fight back aggressively and bring terrorists to justice."

Nielsen, who was sworn in as the nation's sixth DHS secretary last week, said her department would also continue to take steps around the country to fight the threat of similar attacks.

"More broadly, the administration continues to adopt significant security measures to keep terrorists from entering our country and from recruiting within our borders," she said.

The New York Police Department took a man into custody Monday morning who they believe set off an explosive in a subway tunnel, which caused a handful of injuries, mostly to himself.

Police identified the suspect as Akayed Ullah, and authorities said he appeared to be trying to carry out a terrorist attack.