NEW YORK  — The Boston Marathon explosions and their aftermath were captured in chilling images that ran as relentless tape loops of terror online and on TV networks Monday, a sickeningly familiar routine in an age of violence designed for maximum impact.

Broadcast and cable news networks were on the story full time within an hour of the detonations. Screens barely cut away from the scenes. One video repeated dozens of times quickly became iconic: an overhead shot of the race’s finish line with the blast flashing behind spectators on the right, causing one runner’s legs to wobble as he crumpled to the ground.

Another video, taken by Steve Silva of the Boston Globe, showed the first explosion from ground level. As the camera panned over scurrying people and injured lying on the ground, the second blast goes off a short distance down the street.

Whoever responsible made sure it was not only horrific but well-documented. It happened at the heavily populated finish line of the centerpiece event of Boston’s Patriots Day holiday, “almost like New Year’s Eve in broad daylight,” said NBC News’ Brian Williams. It was a place certain to be filled with cameras held by professionals and amateurs alike.

Several times, CBS News ran what appeared to be smartphone footage taken shortly after the first blast “Something just blew up,” a woman said. Then the picture becomes fuzzy as the second explosion is heard.

“Run! Go!” the woman shouts.

Television networks depicted chaos but were restrained in showing gore. One oft-repeated image showed a woman with a bloodied leg being rushed away from the scene in a wheelchair. Through wars, school shootings and terror attacks, it’s a drill TV producers have learned from experience.

One of the most gruesome images, a still photograph taken by Charles Krupa of The Associated Press, showed a man being pushed in a wheelchair. His lower leg was blown away, with bloodied bones hanging down.

The image was sent to Associated Press members in two versions. In one, the leg was leg cropped out and in the other it was shown, said Santiago Lyon, AP vice president and director of photography. Many AP photos are sent directly to news websites with no outside filtering, but this picture was held back so editors could make their own judgments about whether to use it.

“Different markets have different tolerances for violence and gore,” Lyon said. “We’re pretty sure that parts of the world will make good use of it. We didn’t want it to get out in the flow with no human intervention.”

The Atlantic magazine’s website used Krupa’s image but required users to click on a warning before viewing it.

The Huffington Post web site ran several gruesome pictures, including Krupa’s and others with injured people lying on blood-splattered sidewalks. The website’s slideshow was preceded by a printed warning that “the following pictures are extremely graphic and may be disturbing to some viewers.”

The Boston Globe web site clustered video clips of the chaos following the blasts on a separate page. “We’ve had an attack,” one man says on a video. He mutters, “Oh my God, oh my God, oh my God,” as he continues to shoot video of first responders clearing debris.

CBS anchor Scott Pelley told viewers that “there have clearly been cases of amputation in some of the videos.” The network did not show any such footage.

At one point, Fox News Channel’s Shepard Smith was describing an interview that Fox had conducted with a doctor inside Massachusetts General Hospital who told of some gruesome injuries. The video as he spoke showed first responders wheeling gurneys and wheelchairs with the injured, either covered by blankets or without severe injuries apparent. There were also long-distance shots of people being aided on the sidewalk and of bystanders rushing from the scene.

“Every time they do it will scare us, just as it did in the year 2001 in this city,” Smith said. “We ought to give our kids a hug and a kiss, and remind the people next to us that we love them. And remind whoever’s responsible for this that you will not take us down, not on Patriot’s Day, not in Boston, not ever.”

As with most of these breaking situations, there were reports that proved unfounded and injury estimates that changed as the hours wore on. For a brief time, it was believed that there was an explosion at the John F. Kennedy library, but that proved to be an unrelated incident.

In the early hours, there was little active speculation on who might have been responsible. CBS’ Bob Orr noted that experts were not seeing the type of chatter that would have indicated this was a wider-scale event. Jonathan Karl of ABC News talked about the timing — how Patriot’s Day in Massachusetts and the day taxes are collected might have been a trigger.

Television networks quickly made plans for additional coverage, expanding their evening news programs to an hour to cover the story. NBC’s Matt Lauer and ABC’s George Stephanopoulos were heading to Boston for additional coverage.

Social networks were filled with conversations, with celebrities like LeBron James and Paula Abdul offering sympathy to victims. People on Twitter were also urging television networks — and fellow tweeters — to show caution in what they were reporting to avoid inflaming the situation with false details.