It will likely be difficult to determine who blew up the Nord Stream pipelines connecting Russia and Germany in the Baltic Sea last year, Sweden's prosecution authority, which is investigating the incident, said on Thursday.

The September 26 explosions at the pipelines connecting Russia and Germany occurred in the economic zones of Sweden and Denmark, which caused a major gas spillage.

"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," prosecutor Mats Ljungqvist said in a statement.

"There is a variety of information and reports about the sabotage against the gas pipes. The incident has obviously become an open arena for different influence attempts," Ljungqvist said in the statement.

While no conclusion has been drawn, there have been a number of theories as to who blew up the pipelines and how.

Germany has confirmed its investigators raided a ship in January that may have been used to transport the explosives used to blow up the pipelines.

Russian President Vladimir Putin has alleged the United States is behind what he called a "terrorist attack" on Nord Stream pipelines, which was denied by the United States.

Read more:

Nord Stream sabotage and U.S. media

Hersh: U.S. government feeds media 'false' stories on Nord Stream

U.S. highly likely behind Nord Stream blasts, independent investigations needed: Canadian politician

(With input from agencies)