MOSCOW - Russian television on Sunday aired footage of the teenage gunman who killed 20 at his college in Moscow-annexed Crimea calmly mowing down victims while wearing a T-shirt emblazoned with the word "Hate."

A reporter said the security camera footage was broadcast because of widespread speculation about the official account of the massacre on the peninsula annexed from Ukraine in 2014.

The footage confirms that the 18-year-old gunman, Vladislav Roslyakov, dressed to imitate the Columbine high school shooting in the United States, after Russian President Vladimir Putin blamed the crime on "globalisation."

In an apparently new development, Roslyakov is shown talking to another boy, who could be an "accomplice", a television reporter suggested, while investigators said he acted alone.

Officials have questioned how Roslyakov mounted a massive killing spree without outside help.

"After the tragedy there have been so many rumours and so much speculation and conjecture and it's so important to know the truth," a television reporter said.

The footage, aired on Rossiya-1 television, shows the teenager wearing a white T-shirt with the slogan "Hate" as he enters the technical college carrying a sports bag and bypassing security.

At one point he is shown talking to another pupil, who could be an accomplice keeping watch for him, Russian television suggested.

However, a law enforcement official speaking anonymously told RIA Novosti state news agency on Saturday that Roslaykov "prepared alone" and "had planned this since January 2017."

Roslyakov is seen going into the school's canteen with a rucksack and then leaving without it.

Then he is seen going up to the school's first floor empty-handed just before a fire ball and a massive explosion blows out windows and doors on the ground floor.

Roslyakov is then shown walking towards a female teacher in a corridor and gunning her down, before shooting an approaching student.

He wears a shotgun slung on a strap over the "Hate" T-shirt, black trousers and boots with a knife sticking out of one.

His outfit is similar to that of one of the Columbine high school attackers who killed 12 students and a teacher before committing suicide at their Colorado school in 1999.

Forty-seven injured remain in hospital after the attack last Wednesday, a health official told RIA Novosti state news agency on Sunday.