“Your blood boils at what happened,” Modi said. He promised those behind the attack would “pay a very heavy price for their actions.”

India made clear that it holds Pakistan responsible for the unprecedented attack, leading to a fresh round of tensions between the two nuclear-armed rivals. India and the United States accuse Pakistan of supporting and sheltering militants who launch attacks into Indian-held territory, something Pakistan denies.

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Both India and Pakistan claim the Himalayan region of Kashmir, but it has been divided between them for more than 70 years. Since 1989, militants have waged an ­insurgency in Indian-controlled Kashmir, seeking either to claim independence or to join Pakistan.

Thursday’s attack on the security convoy was the worst attack in the history of the militancy. It came as India is preparing for national elections later this spring in which Modi is seeking a second term. A staunch Hindu nationalist, Modi has made a muscular approach to national security a key element of his political brand.

On Friday, candlelight vigils were held across India in memory of those killed. Television channels showed scenes of grieving relatives and images of a room full of coffins awaiting transport, each one draped with an Indian flag.

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Babulal Lamba, a day laborer in the state of Rajasthan, said his 28-year-old son Rohitash was one of those killed. Rohitash planned to come home to celebrate a holiday next month with his wife and 2-month-old child, his father said. “The attack is a security failure of the government,” Lamba said. “We will find peace when the revenge is five times this.”

A Pakistan-based militant group, Jaish-e-Muhammad, asserted responsibility for the attack. Founded nearly two decades ago, the group was designated a terrorist organization by the U.S. government and has been linked to dramatic attacks inside India, including assaults using car bombs.

The group was banned by Pakistan, but its leader, a radical cleric named Masood Azhar, is reportedly based in the Pakistani city of Bahawalpur. Officials in Pakistan deny they support or protect militant leaders like Azhar.

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On Thursday, the U.S. State Department issued a statement condemning the attack as a “heinous act.” It noted that Jaish-e-Muhammad is based in Pakistan and called on all countries to “uphold their responsibilities” to “deny safe haven and support to terrorists.”

The Indian government said Friday that it will launch an effort to diplomatically isolate Pakistan and canceled Pakistan’s preferential trade status, a symbolic blow to its struggling economy.

Ultimately, India will be obliged to respond to the attack with military means, experts said. The difficulty is that such a step risks “a cycle of ­escalation which you’re not sure you can control or manage,” said Shivshankar Menon, India’s former national security adviser.

In 2016, a group of militants stormed an Indian army base near the town of Uri in Kashmir, killing 19 soldiers. In the wake of the attack, the Modi government launched what it called “surgical strikes” across the heavily militarized frontier that separates Indian- and Pakistani-controlled Kashmir. Indian commandos reportedly launched raids on militant hideouts just inside Pakistani territory.

India had taken such actions in the past, security experts said, but the government never previously made them public. Pakistan, for its part, did not retaliate and denied that such cross-border raids even took place in 2016.

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It is only a matter of time before India reacts militarily, said Happymon Jacob, a professor of international relations at Jawaharlal Nehru University in New Delhi and the author of a recent book on India-Pakistan relations in Kashmir.

Jacob said India might decide to repeat the type of strikes it conducted in 2016 against militant camps in Pakistani territory. It could also launch a “fire assault,” he said, which involves increased firing of heavy artillery across the line of control that divides Kashmir.

Such options are fraught with uncertainty because there is no clarity on how Pakistan would react, plus the steps may not have any lasting impact on militant attacks. “I do not see any significant strategic options on India’s part,” said Ajai Sahni, a terrorism expert and executive director of the Institute for Conflict Management in New Delhi. “All the government is going to do now is try to engineer some kind of face-saving event.”

In Pakistan, Thursday’s attack was viewed in starkly different terms. The Nation, an English-language daily newspaper, trumpeted the news on its front page on Friday with a headline saying a “freedom fighter” attack had killed dozens of members of the “occupying force” in Kashmir.

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Pakistani politicians reacted with outrage to Indian accusations that their government was complicit in the attack. “Islamabad makes no gains from such an attack,” wrote Sen. Sherry Rehman, a leader of the opposition Pakistan People’s Party and a former ambassador to the United States. “Instead of a political response to years of brutal repression in Kashmir, hysteria is being amplified against Pakistan.”

Back in Awantipora, the site of Thursday’s attack, the mangled remains of the bus that had been carrying security personnel and shards of glass were still strewn on the highway. All along the route, gun-wielding security forces maintained a tight vigil, and markets remained shut on what is normally a busy stretch of road.

About 10 miles from the site of the attack is the home of the alleged attacker, Adil Ahmad Dar. His mother, Hameeda Bano, said her 21-year-old son joined the insurgency last year. “He had a dream to die for the holy war,” she said.

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