How the Oman tanker attack played out

An oil tanker is on fire in the sea of Oman, Thursday, June 13, 2019.

(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.

    Ongoing investigation

    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.