The number of people killed in the attack on the Boston marathon, which left three dead and more than 260 injured, is expected to rise further.
The Boston Police Department says the death toll could rise to at least 1,000 by Friday.
Police say they believe the attacker carried out the attack using a car bomb.
The bombing occurred shortly before 4pm on the day of the Boston Pride parade, when thousands of people are expected to gather.
The event was held in Boston’s Gayborhood, and was one of the busiest on the Pride Parade route.
The attack occurred a day after the US President Barack Obama called on Americans to join together to show solidarity with Boston in the wake of the attack.
The President has been criticised for not doing more to offer support to the victims and the families of the victims.
Earlier on Thursday, President Trump tweeted that he was “deeply saddened” by the attack and the deaths of the two men killed in a shootout with police.
Mr Trump said the attack “should serve as a warning to those who think that if they try to do the right thing, they will not be punished.”
The President said the “sick and demented people” involved in the attacks were “not the Americans I love, they are the enemies of America”.
The President was responding to a tweet from Mr Trump’s account that said the Boston bombers were “so sick and dementated they would rather die in a fight than be alive in America”.
“The cowardly act of violence on April 15 in Boston, MA will not deter the American people from standing up for our freedom and for the right to life,” Mr Trump tweeted.
Mr Obama on Thursday condemned the attack, saying the Boston bombings represented the “evil that infects our politics”.
“As we mark this tragedy in Boston and throughout our nation, we cannot allow any senseless acts of violence against innocent people to divide us.
We stand together,” Mr Obama said.
Boston Mayor Marty Walsh said on Thursday the US is “shocked and saddened by the horrific and despicable acts of terrorism” that took place on the streets of Boston on April 19.
Mr Walsh said the US “stands with Boston” in the aftermath of the bombing.
The Governor of Massachusetts, Charlie Baker, has declared a state of emergency and is ordering federal troops to help Boston police with the investigation.
Mr Baker said he had spoken to President Trump by phone to express his “deepest condolences”.
“We stand with the people of Boston, as we do every day, in the face of this terrible event,” he said.
Mr Whitehouse told ABC News that the attack was “an act of terror, an act of hate and a cowardly act”.
He said the victims of the bombers had been “victims of a vicious and violent ideology”.
Boston Mayor Martin Walsh said Boston was “shattered” by Boston Marathon attack on April 18, 2017.
The Mayor said that the bombing in Boston had “shaken the American soul”.
“Boston is a city of great diversity and we stand united, united in our grief, in our prayers and in our support for those who were affected by this senseless act of terrorism,” Mr Walsh added.
A witness said the car bomb was made up of nails, wires, and screws, and he saw people being taken away in the street as people were being shot at the scene.
“People were running away, people were screaming, people weren’t running,” witness Eric Wessel told ABC.
“They were in the streets, they were being arrested, they weren’t fleeing the scene, they just wanted to get away.”
A man who saw the explosion said he saw a car explode on the ground and the two suspects running away.
Mr Wessel said he was on his way to work at the time and saw a huge explosion in the middle of the street.
“I just saw this huge explosion, like a fireball, and then I heard this huge bang and then saw this guy coming down the street,” he told ABC Radio Melbourne.
“He was just running, he was screaming, he just wanted everybody to get down on the street, just get down, I just saw a whole bunch of people running.”
Mr Wsel said the man was shot and killed by police.
The second suspect was killed by authorities.
The man who was shot by police said he believed he was being shot for no reason at the moment.
He said he heard two gunshots and saw people running away from the scene as he was shot.
“It was pretty much the same moment as I saw it,” he added.
The two suspects were killed by the police.
ABC/Reuters 3 Police said a car was found at the site of the explosion.
The FBI said they believe there may be more suspects, including two brothers.
Mr Biden also said the men were not involved in a criminal organisation.
He condemned the “horrific” attack and urged