Taipei, Taiwan – When Taiwan seized a Chinese-crewed cargo ship suspected of deliberately severing one of its undersea telecom cables last month, authorities pledged to “make every effort to clarify the truth” of what happened.

Taiwan’s Coast Guard Administration said it could not rule out the possibility that China had deployed the Togo-flagged Hong Tai 58 as part of a “grey area intrusion”.

Recent cases of damage to submarine cables around the island and in Europe suggest that proving sabotage, much less holding anyone accountable, may be no easy task.

Since 2023, there have been at least 11 cases of undersea cable damage around Taiwan and at least 11 such incidents in the Baltic Sea, according to Taiwanese and European authorities.

Taiwanese and European authorities have identified China or Russia – allies that share increasingly strained relations with the West and its partners – as the likely culprits in a number of incidents, though they have attributed several others to natural causes.

In January, NATO launched Baltic Sentry to step up surveillance of suspicious activities by ships in the Baltic Sea.

But so far, authorities have not announced specific retaliatory measures against Beijing or Moscow, though the European Commission has unveiled a roadmap calling for the enforcement of sanctions and diplomatic measures against unnamed “hostile actors and the ‘shadow fleet’”.

Authorities have also yet to criminally charge any individuals or companies despite detaining a number of vessels and crew, including the Hong Tai 58, which was seized near Taiwan’s outlying islands on February 25.

Beijing and Moscow have denied any involvement in sabotaging undersea cables.

“This is what the entire grey zone is about. It’s about being deniable,” Ray Powell, the director of Stanford’s Sea Light project, which monitors Chinese maritime activity, told Al Jazeera.

“You just have to be just deniable enough so that even though everybody knows it’s you, they can’t prove it’s you.”

Subsea cables – which crisscross the globe carrying 99 percent of intercontinental digital communications traffic – regularly suffer damage due to age, environmental changes and marine activities like fishing.

Cable faults are so common – numbering between 100 and 200 each year, according to telecommunications data provider TeleGeography – that industry practice is to build subsea networks with built-in redundancies to ensure ongoing connectivity if one cable breaks down.

These characteristics also make subsea cables a prime target for “hybrid warfare” or “grey zone activities” – low-grade coercive acts that are often opaque and conducive to plausible deniability – according to security analysts.

“Most cable breaks are the result of accidents… anchors may be unintentionally dropped in rough seas or left out for longer than intended. Cables may also break when fishing nets are dragged in the wrong location. What’s more, a ship may not realise it has broken a cable,” Kevin Frazier, a Tarbell fellow at the nonprofit Lawfare, told Al Jazeera.

“The simplest way for a bad actor to break a cable is to make it look like one of the accidents that commonly cause such breaks. Anchors being dragged across a cable is one such cause.”

Barbara Keleman, an associate director at London and Singapore-based intelligence firm Dragonfly, said that the spate of recent cable breakdowns featured tell-tale signs of sabotage despite the relatively large number of failures each year in non-suspicious circumstances.

“If you just look at the data, like how often these incidents are now occurring and how many cables are suddenly damaged at the same time, and you include into that the proximity of some of those ships near those cables, you have statistical deviation which suggests that there is something else going on,” Keleman told Al Jazeera.

The incident involving the Hong Tai 58 came just weeks after Taiwanese authorities briefly detained the Cameroon-flagged Shun Xing 39 on suspicion of dragging its anchor over a section of the Trans-Pacific Express cable, which connects Taiwan with the United States West Coast.

Coastguard officials said they were unable to board the vessel due to bad weather and the vessel sailed on to South Korea.

Industry publication Lloyd’s List said the Chinese freighter turned its automatic identification system (AIS) on and off and broadcast as many as three separate identities.

Enforcing the law at sea is notoriously difficult for not only practical reasons but legal ones as well, including conflicting claims of jurisdiction.

Under the United Nations Convention on the Law of the Sea, ships sailing in international waters are generally subject to the legal jurisdiction of the country under whose flag they are registered.

Within a state’s territorial waters, defined as 12 nautical miles (22km) from shore, vessels are subject to the jurisdiction of that country.

Authorities can, however, exercise “universal jurisdiction” over a ship outside of their territorial waters in a limited number of circumstances, including cases of piracy, “terrorism” and slavery.

Some countries also assert jurisdiction in international waters in cases where a citizen is a victim or perpetrator of a crime.

Even in cases where authorities may have jurisdiction and evidence, it can be hard to make a legal case for deliberate sabotage, said Dragonfly’s Keleman.

“If the investigators or the country’s intelligence services can get a hold of a communication that clearly shows a command for the ship captain to do this, they might have an argument and can try to prosecute,” she said.

“I suspect that’s going to be quite difficult.”

The European authorities’ investigation of the Chinese-flagged Yi Peng 3 following the severing of two subsea telecom cables in November underscored the challenges of responding to acts of suspected sabotage.

AIS data showed the Yi Peng 3 slowing near the two cables – which connected Finland with Germany, and Sweden with Lithuania – around the time of their severing.

Sonar images of the nearby seafloor showed evidence that the vessel had dragged its anchor for as far as 160km (99 miles).

Despite the evidence, European investigators soon hit a diplomatic wall because the ship was flying under the flag of China and was anchored in international waters.

Beijing announced it would investigate the incident itself, though it allowed representatives from Germany, Sweden, Finland and Denmark to board the vessel as “observers”.

In late December, China’s Ministry of Foreign Affairs said the Yi Peng 3’s owner had decided to resume its voyage in consideration of the crew’s physical and mental health and following a “comprehensive assessment and consultation” with European authorities.

China’s Maritime Safety Administration and its embassy in Stockholm did not respond to Al Jazeera’s requests for comment.

Sweden’s Foreign Minister Maria Malmer Stenergard at the time criticised Beijing for not allowing investigators on board to carry out a preliminary investigation.

“Our request that Swedish prosecutors, together with the police and others, be allowed to take certain investigative measures within the framework of the investigation on board remains. We have been clear with China on this,” Stenergard said.

But even if European investigators were dissatisfied, there was not much else that could be done short of causing an international incident, said Jens Wenzel, a Danish defence analyst at Nordic Defence Analysis.

“In international waters, it is quite difficult without the consent of the master, owner/operator or flag state. Within territorial waters the jurisdiction of the coastal state kicks in, which allows for inspection if there is any suspicion of illegal activity,” Wenzel told Al Jazeera.

“In the case of Yi Peng 3, she anchored exactly outside Danish [territorial waters], giving both coastal states Denmark and Sweden difficulties using force to go onboard and without the adequate legislation in place.”

In the months since the Yi Peng 3 left Europe, incidents of cable damage in the Baltic Sea have continued even as NATO has pledged to step up its defence of the region.

They include a December 25 incident involving the Eagle S, a suspected Russian oil tanker flying the flag of the Cook Islands.

The ship dragged its anchor 100km (62 miles), damaging subsea cables in the Gulf of Finland, according to Finnish authorities.

Unlike other cases, Finnish authorities steered the ship into their territorial waters and impounded it.

Three crew members are currently under a travel ban and a criminal investigation is ongoing, although the Eagle S itself was allowed to depart Finland last month.

Herman Ljungberg, a Finnish lawyer representing the owners of the Eagle S, told Al Jazeera that the accusations are “nonsense”, and said that Finnish police had “searched the vessel in and out for nine weeks and found nothing.”

With US President Donald Trump pushing to end Russia’s war in Ukraine, Finland’s intelligence service warned last week that the end of the conflict would free up resources for Russia and its proxies to carry out acts of sabotage.

“The use of proxy operators by various states has recently become a more prominent aspect of both the intelligence and broader influencing scenario. Sabotage operations in Europe linked to the Russian military intelligence service GRU are one example of this,” the Finnish Security and Intelligence Service said in a statement.

“By using intermediaries, Russia seeks to cover its tracks. Russian sabotage operations aim to influence public opinion and the sense of public safety, and to overwhelm the authorities in target countries.”

Russia’s embassy in Stockholm did not respond to a request for comment.

Sea Light’s Powell said acts of sabotage against subsea cables are likely to continue.

“It appears that this is something of a recent trend, and China and Russia and others will do this because they will essentially calculate that the response will not be bad enough,” he said.

“The question then comes down to, how does the international community respond? How does Taiwan respond? What has happened to China or Russia that has yet to send the message that this is so intolerable that it’s not worth doing again?”