The FBI released video of shooter Aaron Alexis walking inside Navy Yard with a gun on the day of the shooting. (Thomas LeGro/The Washington Post)

The video is just 31 seconds long, scarier for what it doesn’t show than what it does. In the first frame, a dark sedan pulls into a parking garage; in the second, a lanky bald man strolls into an office building, catching the door that another pedestrian has thoughtfully held open. In the ninth second, the footage goes from black-and-white to color, and the bald man raises something to his chest. It is a gun. He is going to kill 12 people, but we don’t see that on video.

On Wednesday afternoon, the FBI released footage of Aaron Alexis on the morning he entered Building 197 and began a 69-minute attack that ended with his own death at 9:25.

“We recognized that there is a tremendous public interest in what occurred at Navy Yard,” said Ron Hosko, assistant director of the FBI’s criminal division, in a news conference Thursday. By Thursday evening, the video had received 3.8 million YouTube views, making it the second-most-watched video on the bureau’s channel. The first is the footage of Tamerlan and Dzhokhar Tsarnaev, strolling past a storefront on the morning of the Boston Marathon.

That strolling became a scary image; within hours, bombs at the finish line would kill three and injure more than 200, yet on camera the men suspected of planting them appeared calm.

Last year, Norwegian public broadcasting aired footage of Anders Behring Breivik pulling a white van into an Oslo parking space. The van was rigged with explosives; when they went off, eight people died, and Breivik would go on to shoot and kill 69 more. So a man parking a van became a scary image. In late 1999, the first images of the Columbine High School shooters, sauntering through a still cafeteria after murdering 12 of their classmates and a teacher, were played on a news station. A cafeteria became a place of fear.

Surveillance video released by the FBI Thursday shows the two men they have identified as suspects in the Boston Marathon bombing. (Courtesy of FBI)

The surveillance footage of the Tsarnaevs was released as a call for assistance; the brothers had not yet been identified and officials hoped that, by releasing it and photographs, viewers might recognize the dangerous suspects. But Aaron Alexis had been identified on the day of the shooting at the Navy Yard. He had already been stopped, and killed. The public’s help wasn’t needed. “We weighed the public’s interest and desire to know,” Hosko said at the news conference. But at what point is our desire to know merely prurient? Ten days after the attack, do we already need the film version?

“It was not something that we did unilaterally,” FBI spokesman Paul Bresson said of the decision to release the footage. “It was something we worked with other agencies on” before making the final decision. Victims and families were notified ahead of time, Bresson said.

FBI officials also said that the video was a way of dispelling inaccurate information and quashing conspiracy theories: One persistent report asserted that Alexis had used an AR-15 semiautomatic rifle; in the video, he is clearly carrying a shotgun. Other rumors speculated about a second gunman; in the video, Alexis acts alone. (At least, the conspiracy theorists will note, he is alone in the 30 seconds of footage we have been permitted to see.)

The footage that is now racking up YouTube views — the small slice that officials thought the public should have access to — was edited for our own good, but that might not mean we’re shielded.

“Imagination and people filling in the blanks can be as scary as people actually seeing bodies,” said Elspeth Cameron Ritchie, a Washington-based expert in disaster psychiatry. “We’ve read the coverage, blow by blow, which talks about what he did, where he went.” We know that people are about to die, even if we don’t see it on-screen.

This is why, Ritchie said, trauma psychiatrists recommend against watching videos of violence, or of disaster, such as the World Trace Center towers falling again and again on YouTube.

“I saw the Columbine tape only a few weeks ago, when we were doing a mass-shooting training,” Ritchie said. “I found that very upsetting personally — it found its way into my dreams.”

The video she saw was the full version with audio, not the silent truncated version that was displayed in newscasts or used in later documentaries.

You can’t imagine it. You don’t want to.

There was a fuller version of the Navy Yard video, as well — hours of footage culled to 31 seconds that show nothing but a man in a hallway yet are still nauseating. Someone had to watch all those hours. Someone had to see what came after he ran down those hallways.

“I will tell you,” Hosko said, “that I did see far greater video than was displayed yesterday.”

Sari Horwitz contributed to this report.