Engineers first noticed a drop in pressure in the early hours of Sunday morning and decided to cut off the flow of gas.
A gas pipeline in the Baltic Sea that was shut due to an apparent leak, was likely "caused by external activity," Finland's president said in a statement Tuesday.
The Baltic Connector pipeline, which supplies Finland with gas from Estonia, and also carries telecoms links, was closed on Sunday after a leak was discovered.
"It is likely that the damage caused to the gas pipeline and the telecommunications cable is the result of external activity", said President Sauli Niinistö, stressing that the cause of the leak was not yet clear and that investigations were continuing between authorities in Helsinki and Tallinn.
"What specifically caused the damage is not yet known. The investigation will continue."
NATO Secretary General Jens Stoltenberg wrote on X, formerly Twitter, that he had spoken with Niinistö about the damage to the Baltic Connector and is sharing information with its allies.
When was a problem first detected?
System operators Gasgrid Finland and Estonia's Elering said they noted an unusual drop in pressure in the pipeline shortly before 02:00 EET Sunday morning, after which they shut down the gas flow.
“Based on observations, it was suspected that the offshore pipeline between Finland and Estonia was leaking,” Gasgrid Finland said in a statement at the time. “The valves in the offshore pipeline are now closed and the leak is thus stopped.”
The Finnish operator gave no reason for the suspected leak and said it was investigating the incident together with Elering.
Repairing damage to the pipeline could take "at least several months" depending on the nature of the damage, Gasgrid Finland said before President Niinistö's Tuesday afternoon comments.