HELSINKI (AP) — Finnish police said Wednesday they have launched a criminal investigation into possible sabotage of an undersea gas pipeline between Finland and Estonia that was shut down over the weekend following a leak.

Finland’s National Bureau of Investigation, or NBI, says it has started gathering evidence at the location of the leak, which was detected in Finland’s economic zone Sunday on the Balticconector pipeline connecting the two NATO allies. A telecommunications cable also was damaged.

The purpose of the probe is to establish whether the Balticconector pipeline was damaged intentionally or by accident and by whom. Finnish authorities have already ruled out an operational mishap, saying the damage resulted from “external activity.”

Noting that the analysis was still in an early stage, NBI said “traces have been detected in the seabed” near the leak but didn’t give details.

Outside analysts have speculated on everything from a ship’s anchor hitting the pipeline to an explosion as possible causes. Investigators told reporters on Wednesday that an explosion appeared unlikely.

“The damage appears to have been caused by mechanical force, not an explosion,” NBI chief inspector Risto Lohi was quoted as saying by Finnish public broadcaster YLE. “At the moment we are determining what happened and (who) may have been involved. Considering the situation, we will not speculate, but work to find facts, analyze them and then draw conclusions about what caused the damage.”

The 77-kilometer-long (48-mile-long) Balticconnector pipeline runs across the Gulf of Finland from the Finnish city of Inkoo to the Estonian port of Paldiski. It is bi-directional, transferring natural gas between Finland and Estonia depending on demand and supply.

The 300 million euro ($318 million) pipeline, largely financed by the European Union, started commercial operations at the beginning of 2020. It was shut down on Sunday after operators noticed a drop in pressure in the pipeline.

The incident comes just over a year after the Nord Stream gas pipelines running between Germany and Russia in the Baltic Sea were damaged by explosions believed to be sabotage. The case remains unsolved.

In Brussels, NATO Secretary-General Jens Stoltenberg said he had discussed the latest incident with Finnish President Sauli Niinistö and Estonian Prime Minister Kaja Kallas.

“If it is proven to be a deliberate attack on NATO critical infrastructure, then this will be of course serious, but it will also be met by a united and determined response from NATO,” Stoltenberg said.

On Tuesday, Finnish officials did not comment on whether they suspected Russian involvement.

Kremlin spokesman Dmitry Peskov called the incident “alarming news.”

“We know that there have been dangerous precedents of terror attacks against critical infrastructure in the Baltics, I mean the attacks against Nord Stream pipelines,” Peskov said in a conference call with reporters on Wednesday. “We are looking forward to more detailed information.”

Finnish gas transmission system operator Gasgrid Finland estimated that the repair work would take at least five months. The company said a liquified natural gas terminal in Inkoo has the capacity to deliver the gas Finland needs.

Europe saw natural gas prices hit record highs last year after Russia’s cutoff of most gas supplies during the war in Ukraine. Many European countries have turned to other alternatives including LNG since then to meet their energy needs.

Europe currently has filled 97% of its gas storage capacity for the winter, but security of supply depends on deliveries of pipeline gas and LNG.