The first image of the damaged communications cable between Estonia and Finland was released on Friday by the Estonian Prosecuter's Office and Internal Security Service (ISS).
Martin Arpo, deputy director general of the ISS, told reporters at a press conference that agencies do not usually disclose images of evidence to the public during the investigation phase, but an exception was made due to public interest.
The Prosecuter's Office has been criticized this week for withholding information from the public.
Both agencies said they believe the damage is man-made as natural activity under the sea would not affect the cable this way, newspaper Postimees reported.
Prosecutor Triinu Olev said there are several theories about how the cable was damaged are being pursued.
Among them, it is known that two ships, China's Newnew Polar Bear and the Russian ship Sevmorput, were in the area when the cables broke
"We are currently in the process of drafting a request for legal assistance to China to ask China, in the framework of international cooperation, to assist us and clarify the issues relevant to us in this criminal case," said Olev.
Both agencies said they see little opportunity to communicate with Russia as the legal assistance agreement between the countries is in the process of being terminated. Additionally, Russia has shown little interest in cooperation in the past.
The cable, which is owned by Finnish telecommunications company Elisa, has already been repaired but the investigation continues.
The agencies said there are also plans to raise the cable between Estonia and Sweden in the coming weeks to further investigate the damage.
"At the moment, we have opened criminal proceedings and we are dealing with the Estonian-Finnish communication cable case, but in the coming weeks it will become clear whether there are grounds to extend the criminal proceedings to the Estonian-Swedish communication cable," said Olev.
"In the case of the Estonian-Finnish communication cable, the Estonian-Finnish communication cable has already been found to be broken and damaged, and there are indications of man-made damage. The Estonian-Swedish communication cable is still under investigation, and if we find that this communication cable has also been damaged, and this indicates that it is man-made, then we will have grounds to extend the criminal proceedings to the Estonian-Swedish communication cable," she said.
Estonia is carrying out the investigation into the Estonian-Finnish cable as the damage is located in its zone of economic interest.
It is thought to have been damaged at the same time as the Balticconnector gas pipeline between Estonia and Finland, and a cable between Estonia and Sweden at the start of October.
Finland, which is managing the Balticconnector investigation, has already requested cooperation with China.
This article was updated to add comments from Triinu Olev.
Editor: Helen Wright, Merili Nael