Sweden’s military continues the search for a foreign vessel spotted in Swedish waters last week. Military officials released a picture of the vessel on Sunday, dubbing it as “foreign underwater activity”.

“We don’t know what it is, but we are prepared to use anything necessary to bring the vessel to the surface if we need to,” Swedish military spokeswoman Therese Fagerstedt said to ‘CNN’.

The search has grown into what is now Sweden’s largest domestic military operation since the Cold War. Helicopters, naval ships, and ground troops are being deployed to hunt down the foreign vessel.

“We have support for the analysis work we are now carrying out in the area in the form of ships, planes and territorial army personnel. The area is heavily trafficked, which is why it’s important for us to go public with information about what we’re doing,” commanding officer of the search operation Jonas Wikström said on Sweden’s military’s website.

Swedish military officials declined to voice their suspicions on whom the vessel might belong to or what it may be, but Swedish newspapers are speculating that the foreign vessel is a downed Russian submarine.

Russia, however, has denied the existence of any vessels in Swedish waters. Russian newspapers have even begun to speculate whether a vessel exists at all. Swedish officials are not convinced though.

“They could be mapping the waters in order to be able to navigate them in the event of hostilities,” Swedish Royal Academy of War Sciences fellow Johan Wiktorin said to ‘The Local’. “Or they could also possibly reconnoiter our defense systems.”