HELSINKI — Swedish prosecutors said Thursday a state actor was the most likely culprit for the explosions that incapacitated the Nord Stream gas pipelines in the Baltic Sea last year, an act deemed as sabotage. However, they cautioned that the identity of the perpetrator was still unclear and hinted that it was likely to remain so.

Public prosecutor Mats Ljungqvist from the Swedish Prosecution Authority said in a statement that his office’s investigation is focused on examining if Swedish interests or Swedish security were threatened by the act.

The Swedish authorities are also keen to find out whether the explosions were prepared on their territory. “Our hope is to be able to confirm who has committed this crime” but “it should be noted that it likely will be difficult given the circumstances,” Ljungqvist said.

Last month, a German media investigation quoted unnamed officials as saying that five men and a woman used a yacht hired by a Ukrainian-owned company in Poland to carry out the attack. The Ukrainian government has denied involvement.

The prosecutors’ office gave no estimate on when the Swedish investigation would wrap up.

“This concerns a crime whose circumstances are difficult to investigate. The detonations took place 262 feet under the water on the ocean floor in the Baltic Sea,” Ljungqvist said.

Separately, he told Swedish media that prosecutors’ main line of investigation is on whether a state actor was behind the explosions, given the substantial resources and skills needed to carry out such an attack.

“We aren’t ruling out that there could be nonstate actors capable of doing this,” Ljungqvist told the Swedish news agency TT. “But then we’re dealing with very few companies or groups.”

Two leaks in the Swedish economic zone and two in the Danish economic zone were discovered on the Nord Stream 1 and 2 gas pipelines that run from Russia to Germany through the Baltic Sea, on Sept. 26 and 27 respectively.