(CNN)Russian police are investigating a break-in at a top-secret airborne nuclear command center, nicknamed the "doomsday plane." A cargo hatch was found to have been opened and 39 pieces of radio equipment were reported as missing from inside the aircraft. The suspects have not been identified. Security has now been stepped up at the site, according to the Kremlin, which is investigating the incident. Few details of the aircraft are available, as the plane and its contents are a state secret.Read MoreDmitry Peskov, Putin's press secretary, described the episode as an "emergency situation" in a conference call with journalists and said: "Of course, there will be an investigation, and measures will be taken so that this does not happen again." Peskov did not specify who is responsible for the aircraft's security.The plane had been undergoing routine maintenance to its landing gear rods before the theft, Russian news agency Tass reported, and all equipment was in place at that time. Airborne Command Posts were first nicknamed doomsday planes by the United States, as they are designed for use in the event of nuclear war or a major disaster that would see ground-based command infrastructure destroyed. The most famous models of such specialized aircraft are the US Boeing E-4B, based on the Boeing 747, and the Russian Ilyushin-80, based on the passenger Il-86. According to Tass, the country's armed forces currently have four Il-80s in service.Russia's "doomsday" planes can take off from any civil or military airfield, according to state news agency RIA Novosti. Each has a powerful engine, modern communication facilities and a life support system that enables it to remain airborne for several days. Aside from the cockpit windshield, the plane does not have any windows, so those onboard are not blinded by a nuclear explosion, RIA Novosti said. The tail section houses an outlet cable antenna for communication with submarines.While specifics of the equipment on board the aircraft are not known, it is believed to have everything necessary to control the armed forces, while also accommodating high-ranking passengers with areas for both work and relaxation. Back in October, Tass reported that the planes were set to be upgraded. An aircraft industry source told Tass the updated version would be based on an Il-96-400M quadjet, long-haul, wide-body airliner.The source said the new version would "significantly increase the period of airborne combat duty and to extend the radius for troop command."