NEW YORK -- The FBI is stepping up surveillance of potential terror suspects in the United States who could try to carry out copycat attacks on the homeland, a federal law enforcement official told CBS News senior investigative producer Pat Milton.
The official was not aware of any specific, credible threats of that nature, but said the agency is redoubling its efforts, including looking closely at open working investigations and making sure the FBI knows where certain individuals previously on their radar are located. The European Command has prohibited all unofficial travel to Brussels for American servicemen and their families. Those on official travel must first get the approval of a general officer.
The U.S. Department of Homeland Security said earlier Tuesday that it was closely monitoring unfolding events after bombings in the Brussels airport and subway system and "would not hesitate to adjust our security posture, as appropriate, to protect the American people."
The U.S. Department of Homeland Security sent out a bulletin with information about the attack to law enforcement agencies across the U.S.
Much, if not all of the information in the report, is not classified. A source who's seen the bulletin says much of the information it contains has already been reported publicly, and includes a map of where the attacks occurred.
DHS has reiterated that members of the public should report any suspicious activity in their communities to law enforcement authorities. The department put out a statement Tuesday saying that while there is no specific threat to the U.S., the Transit Security Administration is deploying additional security to airports and transit stations "as a precautionary measure" and DHS remains focused on the threat posed by lone terrorist actors who may lack direct connection to a foreign terrorist organization.
Last week U.S. Homeland Security Secretary Jeh Johnson said Homeland Security officials constantly monitor world events and evaluate whether there is a need to either publicly raise the nation's security posture or issue another bulletin via the government's National Terror Advisory System.
Such a bulletin was issued in December advising the public that federal law enforcement was concerned about the possibility of homegrown violent extremists and terrorist-inspired individuals.
Much like all of Europe's capitals, police in Washington, D.C., said they will also be stepping up their visible presence at all key infrastructure and public interest points in the city in the wake of the Brussels bombings.
The NYPD is increasing visible presence of officers on the streets and in the subway system across the city's five boroughs, reports CBS New York. The Port Authority Police Department is stepping up security at its three major airports. It's also employing high visibility anti-terrorist patrols throughout the PATH system and the World Trade Center site.
"The timing of this event allowed us to hold over the morning tours of duty on all of those units, so we were able to literally double the amount of officers available to us this morning during the morning rush hour on our transit system," NYPD Commissioner Bill Bratton said.
Heavily armed officers and bomb-sniffing dogs greeted commuters, some of whom were stopped to have their bags searched.
In a statement, Gov. Andrew Cuomo laid out some of the additional security measures the state of New York was taking: "The New York National Guard this morning activated its quick reaction force from Fort Hamilton to provide additional security detail at JFK International Airport. The National Guard has 400 service members on duty today in the NYC area. The Guard is adding additional service members to transportation infrastructure hubs downstate, including JFK and LaGuardia airports. The New York State Police will have State Troopers assigned to Penn and Grand Central Stations as well as the Queens Mid Town Tunnel and the George Washington Bridge."
Authorities in Massachusetts said they're stepping up security after the Brussels attacks.
The Massachusetts Bay Transportation Authority said Tuesday that staff will be "stepping up their vigilance and visibility" with Transit Police posted at stations and platforms.
State police said they've increased patrols near transportation centers and are monitoring information related to the terror attacks.
Logan Airport's operator said it doesn't discuss specific security measures but said in a statement they're "multi layered and constantly evolving."
Gov. Charlie Baker took to Twitter to assure residents that security has been strengthened on public transit systems and airports and that law enforcement "will remain vigilant." He also said he's "deeply saddened" at the deaths of victims in Belgium attacks.
In Los Angeles, police presence was beefed up at Union Station, the largest rail terminal on the West Coast. In Washington, D.C., the police chief consulted with the FBI this morning to make sure her officers were doing all they could to help protect the public. The Chicago Police Department said it increased security, as well. K-9 units were stationed on platforms as trains arrived.
Interim Chicago Police Superintendent John Escalante said Tuesday morning that security presence was increased at the city's airports, transportation sites and other high-profile locations. He said there's no specific threat to Chicago but the department wants to be prepared and ready to safeguard critical infrastructure.
Chicago authorities said they're in contact with federal and state officials. Chicago officials are encouraging everyone to be aware of their surrounding and call 911 to report suspicious activity.
The State Department has issued a travel alert through June 20, 2016, following the most recent attacks. It says that U.S. citizens face certain risks travelling to and throughout Europe and warn that "terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants, and transportation."
It cautions Americans to "exercise vigilance" in public places or on mass transportation, to be aware of their surrounding and avoid crowded places, and particularly to show caution during religious holidays and at large festivals or events.