Russian authorities said a 45-year-old suspect had been arrested following the deadly shooting.
Two people have been killed after a gunman opened fire inside a Moscow government services centre.
Four other people --including a 10-year-old girl -- were wounded in the incident on Tuesday in the southeast of the Russian capital.
Moscow Mayor Sergei Sobyanin said on Twitter that a suspect was detained after the "tragedy".
"Doctors are doing everything possible to help the injured," he added.
Russian Interior Ministry spokesperson Irina Volk said that the suspect is a 45-year-old Moscow resident.
The country's Investigative Committee has launched a criminal investigation into the shooting on charges of murder and illegal arms trafficking.
State news agency Tass has cited a police source that the man drew a gun after an argument with a security guard over a face mask.
Masks are mandatory in indoor spaces in Russia, including government service centres, where citizens can apply for passports and other administrative documents.
Fatal shootings in Russia have increased in recent years, prompting calls for a tightening of gun laws by President Vladimir Putin.
In September, a gunman killed six people and wounded 28 others at a university in Perm before being arrested by police. A few months earlier, nine people died in a school shooting in Kazan.