Law Enforcement Ramps Up Security for July 4th Weekend

Law enforcement agencies around the country say they are pulling out all the stops to make sure that this year's Independence Day celebrations are not disrupted by a terror attack. The Department of Homeland Security and the FBI have issued a bulletin to authorities nationwide, warning that ISIS sympathizers may try to stage attacks and police are responding by beefing up security measures.

"Our nation is under threat, our law enforcement, our military are under threat, so we take the threat seriously," U.S. Park Police Chief Robert MacLean says. However, no specific threats have been made.

An ABC News analysis shows that this year alone 40 people with suspected ties to ISIS have been arrested in the U.S. There have been seven arrests in just the last two weeks, including a group of ISIS believers who allegedly had plans to plant a bomb on the George Washington Bridge between New York and New Jersey.

ABC News has checked with police departments in New York, Los Angeles, Philadelphia, Boston and Atlanta and were told that all plan to have full deployments on July 4th. In New York, a sophisticated command center is up and running, with surveillance cameras and police patrols closely monitoring all five boroughs, as well as the waterways and airspace.

"We have a very robust, overlapping, concentric rings of security that we adopt on July 4th," MacLean told ABC News Correspondent Pierre Thomas of preparations for the National Mall.

MacLean said the security at the nine access points to the Mall will be seen and unseen and will focus on protecting law enforcement as well as the crowds. The reason is increasing concern about attacks on military, police and law enforcement itself, MacLean said. ISIS has taken to social media to urge its followers to assault police officers and others in authority.

"We tell our officers on a day-to-day basis to be aware of your surroundings. You as law enforcement are targets, our nation is a target," MacLean said.

And the threats are not just here on U.S. soil. The American airbase at Lakenheath, England has cancelled its July 4th celebration amid reports of worrisome social media chatter revealing specific events and locations.

The base Commander saying he was taking no chances with the safety of his troops.

MacLean and other law enforcement leaders are asking the public to become their eyes and ears this weekend. On the National Mall, visitors can use a two-way text messaging system called Nixle to stay in touch with the police.

"We are able to message to the visiting public and now they are able to message back to us," MacLean said. Mall visitors can text July4dc to 888777 and receive updates on developments and the weather. And, if need be, they can text the Park Police "if there is any type of critical incident law enforcement should be responding to," MacLean said.

Finally, MacLean said, trust your gut. "We use the sixth sense of law enforcement as a great tool for us.

If you believe something is wrong...don't feel ashamed or embarrassed to reach out to a member of law enforcement that is in the area to let them know."

ABC News' Madison Morgan contributed to this report.