More details are emerging about the gun attack at the headquarters of Russia's FSB - the country's domestic security service - on Thursday (December 19) night.
An attack that has left at least one employee dead and five others wounded, before the FSB says it shot and killed the gunman.
Witnesses spoke of their horror.
This man says he saw people running as Russian special forces ran into a large shootout, and several explosions.
This other witness says he saw a police officer laying wounded by a barrier, out of reach of his colleagues during the attack.
On Friday (December 20) morning, mourners gathered to lay flowers at the scene in tribute to the victim of the deadly shooting.
Authorities have not released the name of the attacker or a motive, but it's believed he lived with his mother in a Soviet-era apartment block in the town of Podolsk in the Moscow region.
Some Russian media have named the suspect as Yevgeny Manyurov, a 39-year-old former private security guard.
His neighbors describe him as quiet, and a gun enthusiast, but there were no warning signs about his behavior.
They say the FSB raided his home in the early hours of Friday morning.