HELSINKI (AP) -- A Chinese container ship remains the prime suspect in causing damage last year to a Baltic Sea gas pipeline between NATO members Finland and Estonia, Finnish investigators said Thursday.

It has been over six months since substantial, human-made damage that caused a major drop in pressure was first detected in the Balticconnector pipeline in Finnish economic waters on Oct. 8. Finland and Estonia's gas system operators were forced to shut it down, disconnecting a crucial link between the Nordic and Baltic gas markets for several months.

The pipeline, which runs across the Gulf of Finland between the Finnish town of Inkoo and the Estonian port of Paldiski, was reopened this week after multimillion-euro repair work.

The National Bureau of Investigation, a branch of the Finnish police, said Thursday that it still believes that an anchor of the Hong Kong-flagged cargo vessel Newnew Polar Bear ship, which was on its way to St. Petersburg, Russia, was dislodged and caused the damage detected in Balticconnector.

"Investigation has progressed, and there has been cooperation with the Chinese authorities probing the case," Detective Superintendent Risto Lohi, NBI's head of the investigation, told The Associated Press.

"The main line of investigation has remained unchanged - the cargo ship Newnew Polar Bear and its anchor are considered to be related to the pipeline damage," Lohi said.

Finnish investigators haven't said whether they believe the damage allegedly caused by the Chinese vessel was done intentionally or whether it was caused by incompetent seafaring, as suggested by some experts.

Finnish maritime authorities said at the time of the incident they failed to establish radio contact with Newnew Polar Bear's captain despite several attempts.

Last year, NBI said an initial probe by investigators and experts found a trail about 1.5 to 4 meters on seabed was seen to lead to the point of damage in the gas pipeline. That trail is believed to have been caused by the heavy 6-ton anchor of Newnew Polar Bear, which was later retrieved from the seabed by the Finnish Navy.

"We're probably talking about months before final conclusions," pending further information from technical studies and data from NBI's international partners, Lohi said.

Sauli Niinistö, Finland's former president, spoke with Chinese President Xi Jinping on the Balticconnector case in a video call in January but no details of the talks have been disclosed.

Telecom cables connecting Finland and Estonia, as well as Sweden and Estonia, were also damaged at the same time as the Balticconnector pipeline. Finnish and Estonian authorities believe both cable damages may be connected to the Chinese vessel.

Janne Grönlund, senior vice president at Gasgrid Finland, said Balticconnector was reopened for commercial operation early Monday after gas started flowing from Finland to Estonia. A smaller amount of gas was also flowing in the other direction.

"I'm happy to say that everything has proceeded as planned" since the pipeline's relaunch, he said.

More than a dozen different organisations and companies were engaged in the repairs, which were completed in just over six months. Repairing such submarine infrastructure usually takes one to two years, Estonia's Elering said.

Grönlund said the total cost of the pipeline repair work, performed entirely by remote-controlled equipment at the depth of 60 meters in the bottom of the sea, is estimated at around 35 million euros ($38 million.)

It remains open who will pick up the bill.

Finnish Prime Minister Petteri Orpo last year initiated discussions with European Commission President Ursula von der Leyen on getting financing from the European Union to repair the pipeline. The EU covered 75% of Balticconnector's original construction cost of around 300 million euros.

Following damages to the gas pipeline and data cables, NATO has stepped up its patrols on the Baltic Sea. The alliance has sent minehunters, maritime patrol aircraft, and drones to the region to secure the area and detect suspicious movement near its critical undersea infrastructure.

Finland, an European Union nation of 5.6 million that neighbors Russia, joined NATO in April 2023 after decades of military non-alignment.