(CNN) - The attack on two tankers in a vital Gulf of Oman waterway Thursday comes at a time of heightened tensions between the United States and Iran in the oil-rich region and raises concerns of a potential conflict in the Middle East.
US Secretary of State Mike Pompeo was quick to point the finger at Iran, saying his assessment was "based on intelligence." While Iran denied involvement, the US military on Thursday released a video that it said showed an Iranian navy boat removing an unexploded mine attached to the hull of one of the tankers in an apparent attempt to recover evidence of its participation.
On Friday, however, the Japanese shipping company that owns one of the tankers said it did not believe its ship was attacked by a mine.
The incident bears similarities to an attack on May 12 when four oil tankers were targeted off the coast of the United Arab Emirates in the Gulf of Oman.
Like Thursday's attack, that incident took place near the Strait of Hormuz, a vital shipping route that has been the focal point of regional tensions for decades. About 30% of the world's sea-borne crude oil passes through the strategic choke point, making it a flashpoint for political and economic friction.
Here's how the attacks played out.
The first tanker attack
On May 12, four commercial oil tankers were targeted near the strategic Emirati port of Fujairah in the Gulf of Oman, in what the UAE described as a "sabotage attack."
One was flying a UAE flag, and another the Norwegian flag. The other two were owned by Saudi Arabia, which described the incident as a threat to the security of global oil supplies.
The US blamed Iran for the attack, with US national security adviser John Bolton saying, "I think it is clear these (attacks) were naval mines almost certainly from Iran." He did not offer evidence that Tehran was responsible.
Iran denounced the attack and denied involvement. But the incident came as tensions between the US and its Gulf allies were ramping up amid deteriorating relations.
Days before the attack, the US Maritime Administration issued an advisory warning commercial shipping vessels that Iran or its proxies could be targeting commercial vessels and oil production infrastructure.
The US had also recently deployed the USS Abraham Lincoln Carrier Strike Group and a bomber task force to the Strait of Hormuz in response to a "number of troubling and escalatory indications and warnings" from Iran, a US official with direct knowledge of the situation told CNN at the time.
Following the attack, President Donald Trump approved sending an additional 1,500 US troops to the Middle East as part of a "mostly protective" effort to deter Iranian threats.
Weeks before, Trump had announced that the US would formally designate Iran's Islamic Revolutionary Guard Corps, Tehran's most powerful military institution, a terrorist organization.
The incidents come as Iran is promising to restart elements of its nuclear program, following the US's withdrawal from the nuclear pact last year.
It also comes as Iran and Saudi Arabia continue to fight a deadly proxy war in Yemen. In recent years, Houthi rebels have frequently fired Iranian-supplied missiles into Saudi Arabia -- on Wednesday Houthis struck the arrivals hall of an airport in southwestern Saudi Arabia, injuring 26 people.
On June 6, the initial findings of an international investigation into attacks on the four tankers concluded that a "state actor" was the most likely culprit, but did not mention any state by name.
The UAE, Saudi Arabia and Norway told the United Nations Security Council that there were "strong indications that the four attacks were part of a sophisticated and coordinated operation carried out with significant operational capacity."
Diplomats said the assessment of the damage to the four vessels and chemical analysis of the debris recovered revealed "it was highly likely that limpet mines were deployed."
In a printed statement describing the conclusions, the UAE, Saudi Arabia and Norway said the attacks required trained divers and explosive charges placed under the waterline, near the engines so as to not sink the ships or detonate their cargoes, which indicated a knowledge of the design of the targeted ships. The countries say rapid withdrawal of the plotters by fast boats indicated understanding of the geographic area.
June 13 attack
The most recent attack occurred a week later on June 13, when two tankers -- one carrying oil and the other transporting a cargo of chemicals -- were struck in broad daylight sailing through the Gulf of Oman, near the Strait of Hormuz.
The Norwegian Maritime Agency said that three explosions were reported on board the Marshall Islands-flagged Front Altair oil tanker, which is owned by the Bermuda-based Norwegian company Frontline. The company said that a fire broke out after an explosion and that the cause of the blast was unclear.
A second vessel, the Japanese-owned chemical tanker Kokura Courageous, was "attacked" twice "with some sort of shell," the ship's co-manager, Michio Yuube, said.
The vessels were hit "at or below the waterline, in close proximity to the engine room," said the International Association of Independent Tanker Owners (Intertanko).
"These appeared to be well-planned and coordinated" attacks, it added.
All 21 Philippine crew members on the Kokura Courageous were evacuated, Yuube said. The ship's Singapore-based management company, BSM, said a sailor was injured and the vessel had suffered damage to its hull.
The USS Bainbridge was nearby when the incident happened and a tug ferried crew members of the Kokuka Courageous to it. Images released by the US Central Command showed crew from the Bainbridge assisting the sailors following their rescue.
The 23 crew members of the Front Altair were picked up by a South Korean cargo ship, the Hyundai Dubai, which responded to their distress call.
According to a Hyundai Merchant Marine official, the ship's captain said in an official report that he heard three explosions prior to the Front Altair's distress call. He went outside to the dock and saw the ship was on fire. Two sailors approached the Norwegian tanker on a lifeboat and rescued the 23 crew, bringing them to the Hyundai Dubai, which has now docked in Abu Dhabi.
On Friday, the Japanese shipping company that owns the chemical tanker Kokuka Courageous said it did not believe the ship was attacked by a mine.
In a press conference Friday in Tokyo, the president of Kokuka Sangyo Marine, Yutaka Katada, said he believed "there is no possibility of mine attack as the attack is well above the naval line."
According to Katada, a crew member said the second attack came from a flying shell.
Katada said that all crew members were now back on board the tanker, which is currently tagged to the UAE city of Khor Fakkan, and were working to get electricity fully up and running.
Iran blamed again
Pompeo blamed Iran for the attacks, saying the assessment was based on intelligence but offered no evidence to support his claim.
"It is the assessment by the United States government that the Islamic Republic of Iran is responsible for the attacks that occurred in the Gulf of Oman today," Pompeo said in specially scheduled remarks at the State Department Thursday, as investigations into the attacks were beginning.
"This assessment is based on intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication."
On Thursday evening, the US military released video in support of Pompeo's claims of what it said showed an Iranian navy boat removing an unexploded mine attached to the hull of the Kokura Courageous.
In the video, a smaller boat is shown coming up to the side of the tanker. An individual stands up on the bow of the boat and can be seen removing an object from the tanker's hull. The US says that object is likely an unexploded mine.
Another official told CNN that multiple Iranian small boats had entered the area where the Bainbridge continued to be on the scene, prompting US Central Command to issue a statement saying, "No interference with USS Bainbridge, or its mission, will be tolerated."
Saudi Minister of State for Foreign Affairs Adel al-Jubeir told CNN's Wolf Blitzer that the Saudis "have no reason to disagree" with Pompeo's comments blaming Iran for being behind Thursday's attacks, saying "Iran has a history of doing this."
Iran denies any involvement
Iran Foreign Minister Mohammad Javad Zarif said "suspicious doesn't begin to describe" this latest incident, noting that one of the tankers is Japanese owned and the attack took place as Japanese Prime Minister Shinzo Abe was visiting Iran in an effort to calm tensions between Washington and Tehran.
Alireza Miryousefi, a spokesman for the Iranian mission, tweeted a statement saying Iran "categorically rejects the US unfounded claim" that Iran is behind the attacks and "condemns it in the strongest possible terms."
He added that Iran "expresses concern" over the "suspicious incidents." And he called it "ironic" that the US, which withdrew from the Joint Comprehensive Plan of Action nuclear deal with Iran, was now calling Iran to come back for negotiations and diplomacy.
The UN Security Council privately discussed the latest on the attack but declined to produce any formal reaction.
CNN's Barbara Starr, Vasco Cotovio, and Kara Fox contributed to this report.
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