(Bloomberg) — An investigation into a spate of attacks on oil tankers in the Middle East last month has concluded that a “state actor” is to blame, according to a summary of the preliminary findings of the probe by the United Arab Emirates, Norway and Saudi Arabia.

The investigation found a “high degree of sophistication” behind the May 12 attacks near the UAE port of Fujairah, according to the report. The summary was distributed to diplomats attending an informal briefing at the UAE’s mission to the United Nations.

“The attacks required intelligence capabilities for the deliberate selection of four oil tankers from among almost 200 vessels of all types that lay at anchor off Fujairah at the time of the attacks,” according to the summary. “The attacks required the expert navigation of fast boats” which “were able to intrude into UAE territorial waters and to exfiltrate the operatives after delivering the explosive charges.”

Divers were deployed to attack the ships in a manner that would do damage but not risk a major explosion, according to the presentation.

The investigation looked into the mysterious sabotage of ships including two Saudi oil tankers, an Emirati ship and a Norwegian tanker. The attacks came after the U.S. stepped up economic pressure on Iran in early May by allowing the expiration of waivers that had permitted eight governments to buy Iranian oil.

Envoys from the U.K., the U.S., and Saudi Arabia suggested a probable Iranian involvement in the attacks, according to two diplomats who attended the meeting.

The U.S. had previously blamed Iran for the attacks on the vessels, with National Security Adviser John Bolton claiming those responsible used naval mines almost certainly from Iran. Iran has denied any involvement in the incidents and called for an investigation. An Iranian official at the UN mission in New York said on Thursday that he was skeptical of the credibility of the evidence.

Bolton signaled last week that evidence could be presented to the UN Security Council, and diplomats said that option is still being considered.

Tensions between the U.S. and Iran have surged since President Donald Trump withdrew from the 2015 nuclear accord, reimposed sanctions on the Islamic Republic’s economy and bolstered America’s military presence in the Middle East in response to unspecified threats from Iran and its proxies.

Trump said on Wednesday that he’d “much rather talk” with Iran’s leaders than go to war, and Japanese Prime Minister Shinzo Abe is expected in Tehran next week on what’s being seen as a mediation effort.

The report on Thursday said the UAE, Norway and Saudi Arabia plan to share the evidence with the London-based International Maritime Organization.