Reuters-QA What is known about the Nord Stream gas pipeline explosions

One year on from explosions that damaged the Nord Stream gas pipelines under the Baltic sea between Russia and Germany, the question of who was behind them is unresolved.

On Sept. 26, 2022, Swedish seismologists registered several blasts, some 17 hours apart off the Danish island of Bornholm that ruptured three out of four lines of the Nord Stream system, sending plumes of methane into atmosphere.

Russia's Gazprom said about 800 million cubic metres of gas, equivalent to about three months of Danish gas supplies, had escaped. It took several days for the gas to stop leaking.

Since the blasts occurred in the exclusive economic zones of Sweden and Denmark, both countries are investigating, as well as Germany, where the pipes land.

This is what we know so far:

The multibillion-dollar infrastructure project was built by Russia's Gazprom (GAZP.MM) in two stages - Nord Stream 1 and Nord Stream 2.

Each stage consists of two concrete-coated steel pipelines of about 1,200km in length and more than 1m in diameter, laying at a depth of around 80-110m.

Nord Stream pipelines had a total capacity of pumping some 110 billion cubic metres (bcm) of natural gas per annum, more than a half of Russia's total export capacity.

Gazprom owns 51% of Nord Stream 1, while Germany's E.ON (EONGn.DE) and Wintershall Dea (WINT.UL) have 15.5% each, while French Engie (ENGIE.PA) and Dutch Gasunie (GSUNI.UL) hold 9% each in Nord Stream 1.

The Western owners have written off all their investments.

Nord Stream 2, fully owned by Gazprom and operated by Nord Stream 2 AG, was completed in September 2021 at a cost of $11 billion, but was never put into operation because Germany had cancelled Nord Stream 2's certification days before Russia's invasion of Ukraine on Feb. 24, 2022.

Western companies - Shell (SHEL.L), Germany's Wintershall Dea and Uniper , French Engie and Austria's OMV covered 50% of the NS2 construction costs.

All five have also written off their full financing of NS2, each of about 1 billion euros.

Washington and NATO called it an act of sabotage, while Moscow said it was an act of international terrorism.

Sweden found traces of explosives on several objects recovered from the explosion site, confirming it was a deliberate act.

In July, Germany told the U.N. Security Council that it found traces of subsea explosives on a sailing yacht that "may have been used to transport the explosives".

Germany told the U.N. that trained divers could have attached explosives at the points where the damage occurred to the pipelines at about 70 to 80 meters deep.

So far, no one has taken responsibility for the blasts. Russia and the West have pointed fingers at each other.

Russia's Baltic fleet is headquartered in Kaliningrad exclave near the explosions sites and possess vessels capable of deploying navy divers.

According to a German media report, Russian military ships had been operating in the area prior the explosions.

The Washington Post reported last December that no conclusive evidence has emerged to suggest that Russia itself was behind the attacks, citing intelligence sources.

German General Prosecutor Peter Frank also told Die Welt on Feb. 4 that there was no hard evidence to implicate Russia.

Russia has denied it was behind the blasts and blamed "Anglo-Saxons", Kremlin's description of the U.S. and Britain.

U.S. investigative journalist Seymour Hersh alleged in a blog post in February that the operation was carried out by U.S. navy divers with assistance of Norway but Washington dismissed the report as "utterly false and complete fiction", while Norway said the allegations were "nonsense".

Russia has asked the U.N. Security Council for an independent investigation but failed to win support, except from China and Brazil.

Sweden's special prosecutor in charge of the investigation, Mats Ljungqvist, told Reuters in April that the main assumption was that a state or a state-backed group was behind the attack.

Some intelligence reports cited by the U.S. media alleged a "pro-Ukrainian group" was behind the attack. Ukraine has denied involvement.

German media reported that a group of six people - five men and one woman - boarded the yacht in Rostock, Germany, on Sept. 6, using a forged Romanian passport with a stolen identity.

The Wall Street Journal reported on June 10 that German investigators have identified at least one suspect from Ukraine.

The boat, identified as a 15-metre (50-foot) sailing yacht Andromeda, returned to Rostock in Germany on Sept. 23, according to the investigation by Der Spiegel and ZDF.

After leaving Rostock, the yacht stopped at Wiek, a small harbour on the north coast of Rugen, and then was spotted at a small Danish island of Christianso close to the explosion sites.

It then made a short stop in Kolobrzeg, Poland, on Sept. 19, before returning to Germany. Poland said there was no evidence to suggest that it was used as a hub for the sabotage.

While German investigators are focused on the yacht and the Ukrainian version, according to Der Spiegel and ZDF, German officials had previously warned that it could be a "false flag" operation staged to blame Kyiv.

A day after the explosions, on Sept. 27, 2022, German magazine Der Siegel reported that the CIA had warned Germany in summer 2022 about possible attacks on Baltic Sea pipelines.

On June 6, the Washington Post reported that the CIA learned in June 2022 of a Ukrainian plot and shared the intelligence with Germany and other European countries, the paper added.

German regional broadcasters NDR and WDR reported on March 31 that German security services had received information about the Andromeda yacht shortly after the attack.

Germany's Federal Office for the Protection of the Constitution (BfV) questioned employees of the company that owned the Andromeda in 2022, according to the reports.

BfV informed the German Federal Intelligence Service and both agencies passed their findings to the federal prosecutor.

Germany, Denmark and Sweden told the U.N. Security Council on July 11 they could not say when their investigations would be concluded.

Sweden's special prosecutor in charge of the investigation, Mats Ljungqvist, told Reuters on Sept. 20 he aimed to conclude the investigation before the end of this year.

The German federal prosecutor office told Reuters it could not comment and Danish authorities also declined to comment.

The Nord Stream pipelines have been a flashpoint in an energy dispute between Europe and Moscow since Russia's invasion of Ukraine.

The destruction of Nord Stream pipelines hastened Europe's switch to other energy suppliers, with the United States increasing its liquefied natural gas (LNG) exports.

In March, sources familiar with the matter told Reuters that the pipelines are set to be sealed up and mothballed.

One of the two lines of Nord Stream 2 remains intact but is unlikely to be used to transport gas from Russia while the war in Ukraine continues.

Germany plans to use some of the Nord Stream infrastructure to import liquefied natural gas, including from the United States, according to Der Spiegel.