The video Diamond Reynolds broadcast in the moments after her boyfriend was shot by a police officer didn't save Philando Castile. It didn't remove the bullets or take away the blood that soaked his shirt.

It did, however, serve as a witness. And the cellphone from which it was broadcast was a shield of sorts for Reynolds.

Her watershed video, which was shared around the world, revealed how handheld devices are increasingly being used for evidence-gathering and protection, by citizens and law enforcement alike.

"Live video is a tool of self-defense and sharing about injustice," said Jeff Achen, executive director of UpTake, a St. Paul-based news site that publishes raw and livestreamed video from citizen journalists.

By training cameras on police, legislators and people in power, average citizens can take some measure of control in situations where they once felt helpless, Achen said.

In the first minutes of the video, Reynolds, who was instructed not to move, appeared powerless and weaponless — except for her phone.

"The only thing she had power to do was to document and scream to the world: 'Look what just happened,' " Achen said.

Mobile technology has been used in social movements to chronicle violence and to mobilize supporters since the late 1990s, according to Valerie Belair-Gagnon, an incoming assistant professor at the University of Minnesota, who will teach a class on new media and culture.

Social media and mobile video have provided dispatches from the front lines of the Arab Spring, Occupy Wall Street and the Hong Kong Umbrella Revolution. In fact, Black Lives Matter began as a social media hashtag in 2013 before growing into a full-fledged, in-person movement.

But while cellphone use isn't new, Belair-Gagnon said the devices' ever-expanding reach has the ability to turn one-on-one interactions into large-scale mobilization.

Video, in particular, gives voice — and sometimes tangible proof — to a disenfranchised group, said Toki Wright, who co-wrote the 2011 song "Film the Police" after a police officer fatally shot Oscar Grant in California. The song instructs listeners how to record interactions with law enforcement. "It's vital, 'cause our survival could depend on a video going viral," the lyrics say.

"It's very hard to dispute seeing something with your own eyes and hearing it in real time," Wright said. Filming proves "you aren't a potential criminal. You are a human. You're just a human."

The owner of Soul Food multimedia company, Wright considers the phone a powerful instrument of protection.

"The officer might see it and not want to get in trouble, and it's a thing that proves my innocence in case something happens to me," he said. "It is going to be the thing that keeps me alive."

Some in law enforcement see video as a safeguard, too.

Frank Straub, director of strategic studies for the Police Foundation, a national organization, said officers were initially hesitant about body cameras, but now "want them on all the time."

Police can use footage from body cameras and civilian video to improve officer training and engage in community dialogue.

"The camera allows them to tell the story of it, the community interaction from their perspectives," said Straub, a former police chief in Spokane, Wash. "What we need to get better at is taking the footage and fleshing out lessons."

UpTake, which offers video training to citizen journalists from underrepresented communities, got its start filming protests of the Republican National Convention in St. Paul in 2008. Achen, its director, expects more cellphone camera-wielding citizens to follow Reynolds' lead.

"Livestreaming provides security to folks," he said. "This footage will be out there, and they can't take it back."

For these reasons, Unicorn Riot, a nonprofit alternative media outlet founded in Minneapolis in 2014, uses livestreaming as part of its coverage of protests and vigils.

Livestreaming is a perfect source, Unicorn Riot member Lorenzo Serna said, because you can't censor it.

It also deters police brutality, said Andrew Neef, one of Unicorn Riot's creators. Still, it can't physically protect anyone, as Castile's death proved.

And, in some cases, filming could lead to an altercation with police officers who do not understand people have the right to record, said Belair-Gagnon.

But as cellphone video capabilities improve, Neef said he expects livestreaming's popularity to grow.

"People will yell, 'Pull out your cellphones. Pull out your camera and start documenting,' " said Niko Georgiades, another Unicorn Riot creator. "They want to make sure people either try to stay safe or try to put the word out of what's happening."

Reynolds, speaking to reporters the morning after the shooting, explained her reason for taking out her phone at a moment of extreme distress.

"I didn't do it for pity. I didn't do it for fame," she said.

"I wanted to put it on Facebook and go viral so that the people could see. I wanted the people to determine who was right and who was wrong. I want the people to be the testimonies here."

Sharyn Jackson • 612-673-4853

Danielle Fox • 612-673-4525