(CNN) -- Two large explosions rocked an area near the finish line of the Boston Marathon on Monday, injuring at least six people and spewing wreckage in its wake.

Smoke billowed into the air at Copley Square, turning a site of celebration into a mess of destruction.

Crowds had gathered in the area to watch the runners.

At least four people were being treated at Massachusetts General Hospital.

It was not immediately clear what caused the blasts. Authorities rushed to assist the injured and close off the scene.

Out of an abundance of caution, the Lenox Hotel was evacuated, the Boston Globe reported, as authorities looked into possible security concerns.

Video from the moment of one of the blasts showed some people who appeared to be knocked over and then got up, but the video did not clearly show spectators who may have been closer to the site of the blast.

Mike Baingon, who works at the Atlantic Fish Company, said an explosion took place in front of the restaurant and that he was right by the front door at the time.

The explosions occurred at about 2:45 p.m., more than two hours after the first of the race's nearly 27,000 runners had crossed the finish line, CNN Producer Matt Frucci reported.

The race was halted after the explosions as was subway service into the area.

Troops from the Massachusetts National Guard were assisting police as well.

In New York City, authorities stepped up security as well "until more about the explosion is learned," Deputy Police Commissioner Paul Browne said.

(CNN) -- The Florida police sergeant fired for possessing shooting targets resembling Trayvon Martin defended himself Sunday and said the targets were meant to be used as training aids for "no-shoot" situations.

Sgt. Ron King was fired from his job as a firearms instructor for the Port Canaveral Police Department on Friday after an internal review determined he possessed the paper targets and offered them to fellow officers for use during a firearms training session earlier this month.

"When informed of the basic facts, (I) found the entire situation unacceptable," John Walsh, the interim chief executive of the Canaveral Port Authority, told reporters Saturday. "It is not the type of behavior that I want a police officer to have on both a personal and professional level. I find his conduct intolerable and I demanded that the chief immediately start procedures to terminate this employee."

The paper targets show a faceless black hooded sweatshirt with a bull's-eye on the chest. In one hand is a can of iced tea and in the pocket is a pack of Skittles candy, the same items Martin was carrying when he was shot and killed last year in a case that has drawn national attention.

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King, in a video statement posted online, said he bought the targets because the two items -- the Skittles and iced tea -- were not threatening, which meant the target could be used to help train officers when not to shoot a suspect.

King did not say where he bought the targets, but similar ones caused a controversy when they were first sold online a year ago.

"While others have used it as a novelty, I view it as a tool for scenario-based firearms training," King said. Using "real-life situations" as training scenarios is not uncommon for firearms instructors, he said, and they help teach police how to respond to incidents in the future.

"The only stupid act I performed was to believe that some of my coworkers would be mature enough and care enough to use a bad situation as a learning tool," King said, referring to the Martin case.

Port Canaveral is just an hour away from Sanford, where the 17-year-old Martin was killed in February 2012 as he walked home at night from a convenience store.

George Zimmerman, a neighborhood watch captain, is awaiting trial on a second-degree murder charge in Martin's death. He says he shot Martin in self-defense.

King, who was hired in January 2011, disputed some of the details Walsh gave to reporters, including that he offered the targets to other officers to shoot. He said he showed them to only one, a fellow sergeant, to find out whether he thought they could be useful training aids.

King said he offered one of the targets to the sergeant after the sergeant said his son would "get a kick" out of them. Instead of taking it, he said, the sergeant took a picture and then used the image to file a complaint about King.

"To the Martin family, I would like to apologize again for those law enforcement officials that chose to use your son's death as an element for their personal and political gains," King said. "I assure you that the use of these targets that are in question is to prevent a tragedy from taking place."

On Saturday, Martin family attorney Ben Crump condemned the use of the targets as "absolutely reprehensible."

"Such a deliberate and depraved indifference to this grieving family is unacceptable," Crump said in a statement.