It was a question Richard Martinez could not bring himself to ask. So he listened as a family friend made call after call to agencies in Santa Barbara County, California.

Where was Christopher Martinez?

The father needed to see his only child. He needed to know if what detectives told him was true: that a gunman had murdered his 20-year-old son and five other students celebrating the end of school in the college town of Isla Vista.

Martinez says he will continue working to keep guns out of the hands of the dangerous and mentally ill.

Martinez says he will continue working to keep guns out of the hands of the dangerous and mentally ill.

Martinez learned about social media on the fly, and urged students to hashtag the slogan "Not One More."

Martinez learned about social media on the fly, and urged students to hashtag the slogan "Not One More."

Martinez has taken his case around the United States.

Martinez has taken his case around the United States.

The gunman who killed Christopher had a history of mental issues. Martinez thinks it should have been harder for him to get a gun.

The gunman who killed Christopher had a history of mental issues. Martinez thinks it should have been harder for him to get a gun.

Martinez, a criminal defense attorney, hasn't worked in court in the year since his son's death. Instead, he has traveled to speak against gun violence.

Martinez, a criminal defense attorney, hasn't worked in court in the year since his son's death. Instead, he has traveled to speak against gun violence.

Christopher Martinez intended to become a lawyer, following in his father's footsteps.

Christopher Martinez intended to become a lawyer, following in his father's footsteps.

Martinez became an activist when he took to the podium at a press conference intended for law officers to talk about the Isla Vista shootings.

Martinez became an activist when he took to the podium at a press conference intended for law officers to talk about the Isla Vista shootings.

Richard Martinez with a young Christopher. Martinez has made it his mission to fight gun violence.

Richard Martinez with a young Christopher. Martinez has made it his mission to fight gun violence.

Christopher Martinez, 20, and five other students were shot to death while celebrating the end of school in the college town of Isla Vista, California.

Christopher Martinez, 20, and five other students were shot to death while celebrating the end of school in the college town of Isla Vista, California.

But every call proved futile.

Martinez's friend hung up the phone. One full day after the May 23, 2014, shootings, and still there was no answer about Christopher's body. But there was this information:

"There's going to be a press conference," she said.

"I decided I was going to go there and say something," remembers Martinez. "It wasn't a very rational decision. I just did it."

News cameras packed the front steps of the Santa Barbara County Sheriff's station as local and national media awaited an update on the rampage. The gunman, who had a history of mental issues , had killed himself as police closed in.

Reporters weren't expecting anyone other than a uniformed officer to talk. But the man walking up to the podium wore a button-down shirt and casual jacket.

"I'm gonna try to get through this," the man said. He was tall, striking for his brown hair and nearly completely gray beard, and he quaked as he held the podium.

"Our son Christopher Martinez and six others are dead. Our family has a message for every parent out there. You don't think it'll happen to your child until it does."

Martinez, a criminal defense attorney with a loud and commanding voice that served him well in the courtroom, strained under the weight of an impromptu, passionate and public plea.

"Why did Chris die? Chris died because of craven, irresponsible politicians and the NRA. They talk about gun rights, what about Chris' right to live? When will this insanity stop?"

Reporters stood stunned as Martinez wept.

"We should say to ourselves not one more!"

And with that, he stepped away.

Richard Martinez did not plan this moment. But it was the birth of a movement.

No one else should 'lose a child in this preventable way'

Three simple words: not one more.

They captured Martinez's rage, agony and determination. After his impassioned statement to the media, Martinez fielded dozens of interviews with reporters across the country, including me for CNN just two days after his son was killed. We spoke for more than an hour.

"I don't give a sh*t about a politician's sympathy," he told me. "They need to do something."

He showed me pictures of Christopher in his Little League and basketball uniforms. He carted around trophies Christopher won at a young age.

He wanted me -- and every other reporter -- to understand that Christopher Ross Martinez was not just a shooting victim. He was a gifted athlete, a young man generous with his heart and time, someone who dreamed of following his father's footsteps and becoming a lawyer.

Martinez pledged he would turn no media outlet down, no matter how big or small. The criminal defense lawyer would fight in the only way he knew how: with his words. He channeled his rage and agony and talked as much as he could, knowing the public's attention span was short. He'd seen the pattern: Columbine, Aurora, Sandy Hook. The national dialogue on mental illness and guns lasts only a few days -- weeks, at most. And rarely are the Christopher Martinezes remembered.

JUST WATCHED Isla Vista victim's dad shares outrage Replay More Videos ... MUST WATCH Isla Vista victim's dad shares outrage 03:08

A year has passed now. Martinez remains unbowed.

"Sandy Hook happened and I didn't do anything," he says. "Those parents from Sandy Hook went to Congress and tried to get them to do something and they wouldn't listen. And I didn't listen. A majority of people didn't come forward and back those kids up."

He pauses, looking down at the bench we're sitting on. He has agreed to meet me in a memorial park in the center of Isla Vista. The college town built six benches to remember the six innocent students slain that day. Christopher Martinez's bench bears two bonsai trees, in memory of his interest in Eastern culture.

"Nothing that I thought was important before he died is important to me now," says the father. "Nothing. I don't want anybody else to lose a child in this way, in this preventable way in the 21st century in the United States of America. Nobody should have to bury a kid because somebody who had a gun shouldn't have had one."

Not one more: A hashtag is born

Five days after his son's death, officials at the University of California Santa Barbara asked Martinez to speak to students at a campus-wide memorial at the school's soccer stadium.

When Christopher died, Martinez found a mission. At the UCSB memorial, he found he had an audience of 20,000.

"I wanted to ask kids to do something," but at first, he wasn't sure what. Then he settled on an idea: He would ask them to mail postcards to lawmakers with these words on them: 'Not One More.'

A university driver picked him up, and on the way to the memorial. Martinez rehearsed his speech aloud.

"The driver said, 'Well, the kids won't know what a postcard is. You need to say hashtag.' And I said, 'Well, I don't know what a hashtag is.'"

After arriving at the event, Martinez grabbed an usher who quickly explained Twitter and hashtags. Martinez had never logged onto social media before, but if this was the language his audience spoke, he would use it to get his message across.

He asked the students to mail their representatives a postcard -- or to hashtag 'Not One More.'

He thrust his fist into the air, chanting, "Not one more!"

And 20,000 students stomped their feet on the aluminum bleachers and roared back: "Not one more!"

Martinez still didn't comprehend the impact of a hashtag. But he did understand the power of numbers. Later at his hotel, he asked his son's cousin to check on how many people were tweeting #NotOneMore.

The answer stunned him: "50,000."

JUST WATCHED Tragedy unfolds in Isla Vista Replay More Videos ... MUST WATCH Tragedy unfolds in Isla Vista 02:54

Just hours after his speech, #NotOneMore was trending on Twitter.

Soon, he got a phone call from former New York Mayor Michael Bloomberg's gun control group, Everytown for Gun Safety. The group had started collecting postcards that social media followers could print electronically.

Not 50,000, but 2.4 million.

Martinez had grown up hunting birds on his family's farm and served as a military policeman in the U.S. Army. Now suddenly he was the public face of gun control.

He joined forces with Everytown and began touring the country, delivering 60,000 postcards from Florida constituents to Gov. Rick Scott and U.S. Sen. Marco Rubio. Both happen to be Republicans but Martinez doesn't care about political affiliations. He lumps them all in one monolithic, unmoving group.

He traveled on to Harrisburg, Pennsylvania, to deliver 27,366 postcards from Pennsylvania residents to Gov. Tom Corbett's office and 1,318 postcards to Rep. Charlie Dent. None of the lawmakers met with him and their staff viewed him "as an annoyance, frankly," he recalls.

Martinez went to five states. He realized he couldn't stop.

The man who once thrived in a courtroom hasn't stepped back into one since his son's death.. "I have a sense of urgency. I feel the longer it takes us to get common sense changes done to reduce gun violence in this country, the more kids like my son are going to die."

How many more?

There have been more.

Not one more. Incredibly, it's hard to even know how many more.

Less than two weeks after the Isla Vista mass shooting, Martinez's phone buzzed. It was the afternoon of June 5 and a mentally unstable gunman had opened fire on the campus of Seattle Pacific University. The news propelled Martinez back in time.

JUST WATCHED How a student stopped the Seattle shooting Replay More Videos ... MUST WATCH How a student stopped the Seattle shooting 02:55

A deranged gunman. A campus. Innocent students.

Paul Lee was gunned down at age 19 ; two other students were wounded. Law enforcement says the gunman planned to kill as many as possible. A student tackled him as he reloaded his shotgun. His lawyer later said he had a history of mental illness.

It is not clear how many mass shootings have occurred in the year since Christopher Martinez died. The FBI defines a mass killing as a single incident in which at least four people died at the hands of another. But the FBI's Supplementary Homicide Report reflects only the numbers voluntarily submitted by local police agencies, making it incomplete.

An FBI study released last fall examined "active shooter incidents," described as "individuals actively engaged in killing or attempting to kill people in populated areas." The report focused on fluid incidents in shootings like Isla Vista and Sandy Hook. The study reported 160 incidents from 2000 to 2013, with 486 people killed and 557 wounded. It also found the rate of these shootings has risen in the past few years. In the first half of the years studied, the average annual number of incidents was about one every two months. In the second half of the study, the rate increased to more than one incident a month.

News reports examined by CNN show about 30 incidents of mass killings since May 23, 2014 -- 21 of them caused by guns.

But left off that list are incidents like the one at Seattle Pacific University where one person was killed and others were wounded. Richard Martinez considers it, too, a mass shooting.

As he absorbed the news that day, he turned his car around and drove immediately to the local news station near his house.

"I need to talk to someone," he told the person at the front desk. Then, as calmly as he could, Martinez spoke to a local news reporter about "stopping this madness."

Success but not solace

Four months after Christopher's death, California Gov. Jerry Brown signed legislation that allows law enforcement or family to temporarily seize guns from people determined by the courts to be a threat to themselves or others. The family of the gunman in the Isla Vista rampage had sought help for him but he was still able to own guns. Christopher's father, with another victim's father, met with lawmakers to lobby for the legislation.

JUST WATCHED Missed warning signs before UCSB rampage Replay More Videos ... MUST WATCH Missed warning signs before UCSB rampage 02:14

Martinez stumped and helped pass I-594, a measure that expands background checks so all gun transfers, beyond the initial point of sale, must go through a universal background check.

And he traveled to multiple states in opposition of NRA-backed bills that would have eased gun restrictions on college campuses.

He says the NRA is not his enemy. While he views the gun lobby's leadership as more extreme than its membership, he believes its members, like voters, understand no one wants to be the next Richard Martinez.

"I'm not really trying to persuade anybody in particular. My approach is put the best case from my point of view, the gun safety movement point of view, before the voters and let them decide."

A spokeswoman for the NRA would not make an official comment on Martinez's efforts, despite numerous requests from CNN. She did, however, express sympathy for him and other families whose loved ones have been touched by gun violence.

Martinez doesn't want anyone's sympathy. He finds solace in travel that has been relentless, by choice. It has led to meetings with other parents who've experienced the same kind of loss, like Roxanna Green, whose 9-year-old daughter was shot to death with five others on January 8, 2011, by a gunman trying to assassinate U.S. Rep. Gabrielle Giffords.

Roxanna Green gave Martinez a purple wristband bearing her daughter's name, Christina Taylor Green. He gave her a blue one that says Chris Martinez on one side and Not One More on the other. More than a dozen wristbands cover Martinez's wrist, so many that the names blur into one another.

Martinez also wears a leather band with #NotOneMore emblazoned on it, created by fashion designer Donna Karan. Actor Hugh Jackman snapped a picture of himself with it and asked his followers to support #NotOneMore.

Martinez is grateful to the celebrities but says those efforts alone won't achieve what he wants, which is to keep guns out of the hands of the dangerous and mentally ill.

That uphill climb has been the only effective numbing agent for this father.

"If I could trade all the remaining days I have left in my life for one more day with my son, I would do it. I'm moving and doing things and trying to make his death mean something, but nothing I can do will ever make up for it."

Nothing can fill the hole of every birthday, every holiday, every year that he ages and his son remains forever 20 years old.