03/29/2012 04:46 EDT | Updated 05/29/2012 05:12 EDT

New video tape in Trayvon Martin case challenges story from police, Zimmerman

WASHINGTON - A new police surveillance video showing the killer of an unarmed black teen sporting no obvious injuries despite claims of self-defence has heightened calls for the man's arrest, with Trayvon Martin's grief-stricken mother calling it "the icing on the cake."

"Thank God for surveillance video," Benjamin Crump, the lawyer for Martin's family, told "CBS This Morning" on Thursday, "because obviously there was a conspiracy to cover up the truth and sweep Trayvon Martin's death under the rug."

Crump said the video refutes the claims of police and George Zimmerman that the 140-pound Martin, 17, beat him up so badly his nose was broken, and that he shot the boy in the chest in self-defence on a rainy Florida evening a month ago.

"This certainly doesn't look like a man who police said had his nose broken and his head repeatedly smashed into the sidewalk," he said.

Sybrina Fulton, Martin's mother, said the video was yet further proof that Zimmerman's story doesn't add up.

"This video is the icing on the cake," she said. "This is not the first part of the evidence that they have had. They have had the 911 tapes and they have also had witnesses. This video is clear evidence that there is some problem with this case and he needs to be arrested."

Zimmerman's lawyer, Craig Sonner, argued the video, obtained by ABC News, was inconclusive and called the footage "very grainy."

He pointed out that four hours had lapsed between the time of the slaying and when his client was captured on video being led into police headquarters in Sanford, Fla. Zimmerman had received first-aid during that time and had been "cleaned up," he said.

In the video, Zimmerman's head and face are clearly visible, and there appear to be no obvious signs of any external injuries. There's also no evidence of blood on the front of his T-shirt, often the byproduct of a broken nose.

The developments — dubbed "George Zimmerman's crumbling story" by the Washington Post on Thursday — are the latest in the heartbreaking case that has riveted a country that grapples with ever-simmering racial tensions.

Martin's parents believe Zimmerman, patrolling his neighbourhood in an SUV, deemed the boy suspicious because of the colour of his skin as the teen walked through a Florida gated community wearing a hoodie.

The case has become a lightning rod for many African-Americans, prompting them to relive years of grievances about the troubled ties between police, the courts and the black community.

Protests have been held across the country, with Americans ranging from professional athletes to lawmakers donning hoodies while demanding Zimmerman's arrest. One hoodie-wearing state legislator in Illinois, Democrat Bobby Rush, was kicked off the floor on Wednesday in an incident that caused a mini-uproar all its own.

In the U.S. capital on Thursday, yet another demonstration was held, this one outside of the American Legislative Exchange Council, a right-wing group that has long promoted laws like Florida's Stand Your Ground legislation with the help of its allies in the National Rifle Association.

"Pull back the curtain on these ghostwriters who write kill-at-will laws, who poison our communities and suppress the vote," one protester told the crowd in downtown D.C.

Sanford police have said they had no grounds to charge Zimmerman because of the law, which essentially allows people to use deadly force if they believe they're being threatened.

Martin's parents have argued, however, that if anyone was within his rights under Stand Your Ground, it would have been their son if it's true he attacked Zimmerman as he walked back to the home of his father's girlfriend following a visit to a nearby 7-Eleven.

But they doubt their boy — described by friends, family and his teachers as gentle and non-confrontational — aggressively confronted Zimmerman at all.

Tracy Martin, Trayon's father, recounted what a detective told him had happened in the days following his boy's slaying.

The detective said Martin, after noticing he was being followed, appeared from behind a building in the gated community, approached Zimmerman and demanded: "What's your problem, homie?" Martin recalled in an interview with Washington Post reporters and editors.

When Zimmerman replied that he didn't have a problem, the detective told Tracy Martin, his son replied: "You do now" and proceeded to attack him.

"That was bull," Tracy Martin said. "No way. At that point, I knew there was something terribly wrong."

One of the lead investigators on the case also doubts Zimmerman's version of events and wanted to charge him that night with manslaughter. Chris Serino was over-ruled by the Florida attorney's office.

Richard Kurtz, the funeral director who prepared Martin's body, has also weighed in, saying there were no signs of injuries on the boy's hands that might have suggested he'd punched someone.

"The only thing that I was able to see was the gunshot wound," Kurtz said. "I could not see evidence like he had been punching somebody as the news media say he was punching .... It just did not add up to me."

A 911 call made by Zimmerman that night has also done little to bolster his story. The dispatcher is heard telling him not to follow Trayvon, but he ignores the instruction. Martin, meantime, was on his cellphone with his girlfriend, telling her someone was following him and that he was frightened.

Zimmerman can be heard running after Martin on the tape, telling the dispatcher at one point in short breaths: "These ...(expletive), they always get away."

In recent days, details have emerged about Martin being suspended from school three times, including for having a baggie with marijuana remnants in this backpack.

But Zimmerman's past brushes with the law were arguably more serious. He was charged in 2005 with resisting arrest with violence and battery of a law enforcement officer, and his ex-fiancee also successfully filed for a domestic violence restraining order against him.

Neighbours have also complained that he had a habit of targeting young black boys in his neighbourhood watch duties.