A suicide bombing in the offices of Russia’s Federal Security Service in Arkhangelsk in northern Russia has left three injured.

Investigators said a 17-year-old local male had been carrying the bomb in a bag and detonated it on Wednesday shortly after entering the FSB’s offices in the city, which is just south of the Arctic circle.

A CCTV still published by Russian media showed the suspect in a heavy coat reaching with his right arm into a black backpack shortly before the blast. No motive was given for the attack, but Russia’s Investigative Committee opened a terrorist investigation.

The FSB is Russia’s main security agency and manages the country’s border guard service, counterterrorism and counter-intelligence activities. It is viewed as the main successor to the Soviet-era KGB and inherited the Lubyanka headquarters in Moscow. However, there are significant differences between the agencies.

While attacks on Russian law enforcement are not unheard of, they have mostly taken place in the North Caucasus region, where the country is battling a diminished Islamist insurgency and previously fought two wars in Chechnya. Islamic State claimed responsibility for a series of coordinated attacks in Chechnya this August that included children as young as 11. They were recruited over social media.

The motive for Wednesday’s attack is unclear. Some Russian media pointed to a post on the Telegram messaging service shortly before the blast. In it, a user who identified himself as an anarchist, said he would carry out an attack in Arkhangelsk because the FSB “fakes cases and tortures people”.

The FSB was accused of brutally interrogating members of leftist political groups before elections and Russia’s World Cup. But any ties between that user and the bomber have not been confirmed.

Russia’s Investigative Committee has said it is still investigating the motive for the attack. A spokesman told the Interfax news service that the agency’s head “has ordered that the motive of the crime committed by the teenager and its causes and circumstances be established swiftly, and that his social circle, family situation, and relations be identified and analysed”.