Thousands of people were injured in Lebanon after pagers used by the armed group Hezbollah to communicate exploded almost simultaneously across the country on Tuesday.

At least nine people were killed in the explosions, while hundreds more were rushed to hospitals across the country. Lebanon’s Health Ministry says at least 2,750 people were injured, 200 of them seriously.

It is unclear how the attack, which appears highly sophisticated, occurred, although Hezbollah has blamed its opponent Israel. Israeli officials have so far declined to comment.

This is what we know so far.

The explosions began around 3:45pm local time (1:45pm BST) on Tuesday afternoon in southern Beirut and several other areas in Lebanon.

Witnesses reported seeing smoke coming from people’s pockets, followed by small explosions that sounded like fireworks and gunfire.

In a clip, surveillance footage shows an explosion taking place in the pocket of a man standing at a store checkout.

According to Reuters news agency, the explosions continued for about an hour after the first explosions.

Shortly after, dozens of people arrived at hospitals across Lebanon, with witnesses reporting scenes of great confusion.

A source close to Hezbollah told AFP that two of the dead were the sons of two Hezbollah lawmakers. They also said the 10-year-old daughter of a Hezbollah member was killed.

According to Reuters, citing a source, Hezbollah leader Sayyed Hassan Nasrallah was not injured in the explosions.

Analysts reacted with shock to the scale of Tuesday’s attack, saying Hezbollah prides itself on its security measures.

Some suggested that a hack could have caused the pager batteries to overheat, causing the devices to explode. Such an act would be unprecedented.

However, many experts believe this is unlikely, as images of the explosions do not show that the batteries overheated.

Some analysts say a supply chain attack, where the pagers are manipulated during production or transportation, is more likely.

Supply chain attacks are a growing problem in the world of cybersecurity. There have been many high profile incidents recently where hackers gained access to products while they were still in development.

But these attacks are usually limited to software. Hardware supply chain attacks are much rarer because they get their hands on the device.

If this was indeed a supply chain attack, it would have been a massive operation to somehow covertly tamper with the pagers.

A former British Army munitions expert, who wishes to remain anonymous, told the BBC The devices are said to each contain 10 to 20 grams of military-grade explosives hidden in a fake electronic component.

According to the expert, this would have happened with a signal, a so-called alphanumeric text message.

So far, no one has claimed responsibility, although Lebanon’s prime minister and Hezbollah have blamed Israel.

Prime Minister Najib Mikati said the explosions represented a “serious violation of Lebanese sovereignty and a crime by all standards.”

In the statement accusing Israel of being behind the attacks, Hezbollah said it held Israel “fully responsible for this criminal aggression, which also targeted civilians.”

“This treacherous and criminal enemy will certainly receive his just punishment for this sinful aggression, whether he expects it or not,” the report added.

Israeli officials have not yet responded to the allegations, but most analysts agree that it is likely that Israel was behind the attack.

Professor Simon Mabon, head of the Department of International Relations at Lancaster University, told the BBC: “We know Israel has a precedent in using technology to track its targets,” but he called the scale of this attack “unprecedented”.

Lina Khatib of the UK-based Chatham House said the attack indicated that Israel had “deeply infiltrated” Hezbollah’s “communications network”.

Hezbollah makes extensive use of pagers in its communications.

Mobile phones have been discarded for a long time because they are simply too fragile. This was already evident in 1996 when Israel killed Hamas bomber Yahya Ayyash.

But a Hezbollah official told the AP news agency that the beepers were a new brand that the group had not used before.

Emily Harding, a former CIA analyst, said the security breach was deeply embarrassing for Hezbollah.

“A breach of this scale is not only physically damaging, it will also cause them to question their entire security apparatus,” she told the BBC.

“I expect they will conduct an intensive internal investigation, which will distract them from a potential fight with Israel.”

Hezbollah is allied with Israel’s arch-enemy in the region, Iran. The group is part of Tehran’s Axis of Resistance and has been engaged in a low-level war with Israel for months, with regular exchanges of rockets and missiles across Israel’s northern border. Entire communities have been displaced from both sides.

The explosions came just hours after Israel’s security cabinet made the safe return of the inhabitants to the north of the country an official war objective.

Prime Minister Benjamin Netanyahu told a US official that Israel “will do whatever it takes to ensure its security.”

Earlier on Monday, Israel’s internal security service said it had foiled an attempt by Hezbollah to assassinate a former official.

Despite the ongoing tensions, observers say both sides have so far tried to contain hostilities without crossing the line into full-scale war. But there are concerns the situation could spiral out of control, with Hezbollah already threatening to respond to Tuesday’s explosions.

Additional reporting by Frances Mao