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 has not only 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 cops, 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 where 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 police in, so after a standoff that lasted an hour and a half, police removed the door. Police accused the retired corrections officer and former Marine of coming at them with an axe. 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 was also captured from the police Tasers.

The Westchester District Attorney declined to comment on the case, saying just 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.”