MIAMI – Kenneth Chamberlain Jr.’s foray into activism started with a Facebook post.

Someone placed a petition demanding the arrest of Trayvon Martin’s killer on Chamberlain’s Facebook wall, and that got the New York behavior counselor thinking: What about his own dad’s unprosecuted killing?

“I signed Trayvon’s petition, sat back and thought, ‘Well, maybe I should do a petition,’” he said. “I did and got 1,000 signatures. I was happy with 1,000.” 

Then came more: Five months after Chamberlain’s father was killed in his home by a White Plains, N.Y., police officer, Chamberlain not only has 208,000 online signatures — he got a grand jury investigation as well. 

Across the nation, experts say people disillusioned by the criminal justice system were galvanized by the Trayvon Martin case and took to the Internet to demand that police and prosecutors take a second look at questionable shootings. From New York to Chicago, Atlanta, North Carolina and elsewhere, people whose relatives were killed by police, zealous security guards or neighbors are inundating law enforcement officials with online petitions, calls from attorneys and rallies. 

With Trayvon’s killer, George Zimmerman, now facing second-degree murder charges, cases that activists say would otherwise have been swept under the rug are gaining new momentum. Social media tools allow anyone to start a petition and keep in touch with the people who signed. Experts say that has played a key role in spreading the word about other killings and helped empower victims, who are often poor and black.

The phenomenon has lifted the veil on dozens of questionable shootings around the nation in which police or prosecutors carried out lackluster investigations or were perceived to have protected law enforcement, activists said.

“This is a message to law enforcement: Families are no longer powerless,” said Steven Biel, director of SignOn.org, the online petition site associated with MoveOn.org. “These petitions offer a way to send emails and organize people in an ongoing way. That’s the most exciting thing: It’s not just petitions, but organizing rallies and making sure the targets understand this is not just a bunch of people clicking a mouse.”

Kenneth Chamberlain Sr., 68, was killed Nov. 19 after police went to his apartment responding to a medical alert alarm. An agitated Chamberlain would not let police in, so after a standoff that lasted 90 minutes, police removed the door. Police accused the retired corrections officer and former Marine of coming at them with an ax. Officers Tasered him, shot him with a beanbag shotgun and then killed him with a live round.

The audio of the entire episode was recorded by the medical alert company, which caught the officers using racial epithets and the elder Chamberlain telling a black police officer: “Black officer, why are you letting them do this? Why are your guns drawn?” attorney Randolph McLaughlin said. Video also was captured from the police Tasers. 

The Westchester District Attorney declined to comment on the case, saying only that it is being presented to a grand jury. The five-month delay was necessary to gather forensic reports and other evidence, spokesman Lucien Chalfen said.

“It’s been investigated from the beginning,” Chalfen said. “It did not take a long time to take to a grand jury. It’s within the margin of error for the appropriate length of time.”

Although Chamberlain’s shooting has stark differences from Trayvon’s, the two have been likened as incidents in which media attention and activism are credited for reopening investigations.

“They are killing young men in Florida and killing old men in New York, but there are significant differences in the cases,” said McLaughlin, an attorney with the law firm Newman Ferrara in New York. “There are also similarities: Trayvon was walking while black, while Mr. Chamberlain was just living while black.”

Treka McMillian was inspired by the Chamberlain online drive to start a petition of her own. Her goddaughter, Jasmine Thar, a 16-year-old high school sophomore, was killed two days before Christmas in Chadbourn, N.C., when a man across the street said his rifle went off by accident.

The bullet struck Jasmine, McMillian and a 17-year-old. No arrests have been made.

“I don’t care if it was an accident — still someone should be held accountable: Someone shot somebody,” said McMillian, a college women’s basketball coach. “You don’t want to make it about race, but you have to think that if a black person shot two kids, they’d be arrested.” Jasmine was black, and the shooter is white.

The Justice for Jasmine petition on SignOn.org now has more than 70,000 signatures. McMillian recently learned that the FBI will investigate the matter, and the district attorney agreed to meet with the dead teen’s family.

“I didn’t want it pushed under the rug and not dealt with,” she said.

In the Bronx, activists are hoping the shooting of an unarmed 18-year-old in his grandmother’s apartment gains the kind of momentum that Trayvon’s killing did. The Bronx district attorney is investigating why police followed Ramarley Graham to his apartment, knocked down the door and killed him, but no arrests have been made in the February shooting.

Graham was killed in the bathroom, where he was disposing of marijuana. The case has brought new attention to the New York Police Department’s “stop and frisk” policies that activists say inordinately target blacks and Hispanics.

“I think overall there are people concerned and frustrated with the criminal justice system for a variety of reasons,” said Jon Perri, senior organizer for criminal justice at Change.org. “A lot of people feel that if they don’t have money, they have nowhere to turn. This platform is opening the floodgates.”