Online warning claims ISIS has terror cells in 15 U.S. states

A terror threat once thought to be contained overseas is here on United States soil. A new warning coming from someone claiming to be with ISIS says 71 militants are living in 15 states, specifically naming five -- Virginia, Maryland, Illinois, Michigan and California.

This comes after Sunday's shooting in Texas where two men opened fire outside a contest featuring cartoons of the Prophet Muhammad.

ISIS is claiming responsibility for the attack, and if true, it would be the first time the extremists carried out an attack in the United States.

"This is still under investigation by the FBI and other members of the intelligence community to determine any ties or affiliations that these two individuals may have had with ISIL or other terrorist organizations," said White House Press Secretary Josh Earnest.

National security experts are responding to Wednesday's warning, but are skeptical.

"I think it's likely true that there are terror cells," said Paul Hamill of the American Security Project, a non-partisan public policy and research organization dedicated to national security issues. "What I don't think is true is that they are linked directly with ISIS."

He believes it is a matter of when, and not if an attack will happen. But he believes the threat is from small independent cells that sympathize with the terror group.

Hamill's strategy in tamping down the lone wolves starts here at home.

"I think what we should be seeing is the local police taking the lead in this," said Hamill. "Connecting the police with the communities, patrolling, talking to local communities, gaining the trust of local communities, especially working with mosques, which will be big thing."

In the meantime, the state department is offering rewards of several million dollars for information on four ISIS leaders.

The U.S.-led coalition continues to conduct airstrikes on ISIS targets overseas.

The U.S. ambassador to the United Nations Samantha Power said the battle against the group has been slow going, but progress is being made.