Former U.S. Secretary of Homeland Security Michael Chertoff said earlier Friday evening it is too early to know exactly what was happening in Paris.
Social media posts from purported ISIS supporters could indicate that "there was a group waiting for this, but it could be a group watching," Chertoff said in an interview with MSNBC Friday night.
"I don't think we can say this proves anything, but again it supports the idea that it's terrorism," Chertoff said.
The Associated Press has reported at least 120 people were killed in Paris, spanning six sites across the city.
John Cohen, a former Homeland Security Department counterterrorism coordinator, say the presence of multiple attack scenes at the same time suggested a coordinated effort to "send a message" and raises immediate terror concerns, including for other cities in Europe and potentially the United States as well. He said both Al Qaida and ISIS have relied on the strategy of coordinated attacks in the past.
President Barack Obama is calling the attacks on Paris "outrageous attempt to terrorize innocent civilians" and is vowing to do whatever it takes to help bring the perpetrators to justice.
Speaking to reporters at the White House, Obama said he would not speculate about who was responsible.
He called the attacks a "heartbreaking situation" and an "attack on all of humanity."
Obama was briefed on the attacks Friday by his counterterrorism adviser Lisa Monaco.
U.S. Secretary of State John Kerry is describing the attacks in Paris as "heinous, evil" and "vile," calling them "an assault on our common humanity."
Kerry says the U.S. embassy in Paris is "making every effort to account for the welfare of American citizens in the city."
The State Department says U.S. citizens can contact 1-888-407-4747 (from the U.S.) or 202-501-4444 (from other countries) for assistance.
Kerry says the U.S. stands ready "to provide whatever support the French government may require."
Kerry was speaking from Vienna, where he is scheduled to attend talks Saturday on the crisis in Syria.
Vice President Joe Biden calls the attacks "heartbreaking" and "outrageous" and says, "Such savagery can never threaten who we are."
New York Gov. Andrew Cuomo says the One World Trade Center spire will be lit blue, white and red in honor of dozens killed in the Paris attacks.
Cuomo says the 408-foot (125-meter)spire will be illuminated Friday night and in the days to come. The governor says the act shows New York will stand with the people of France.
New York City officers have been deployed to various parts of the city, including French government buildings. Heavily-armed officers stood outside of the French Consulate in Manhattan as passers-by brought flowers.
Police have stressed there is "no indication that the attack has any nexus to New York City."
Massachusetts State Police say they're increasing security around the area of the State House following the deadly attacks in Paris.
In addition, State Police said Friday they are monitoring intelligence at Logan International Airport in Boston.
State Police troop commanders also are directing on-duty troopers to have a heightened awareness of potential suspicious activity within their patrol areas.