BEIRUT—As new details emerged about twin suicide bombings near the Iranian Embassy here, Lebanese officials described an outburst of violence that reveals the resurgence of al Qaeda-inspired groups in their country, a toxic byproduct of the Syrian war.

The attack on Tuesday killed 25 people and wounded 147, but was blocked from reaching its target. The second suicide bomber was prevented from ramming his explosives into the Iranian embassy after a water truck was abandoned at the embassy's gate by frightened bystanders who fled their vehicle after the first bomber, who approached on a motorcycle, detonated his explosives, security officials said on Wednesday.

Behind the attack is a recent shift by radicalized Lebanese Sunnis to join the region's jihadist movement—for years dominated by other Arab nationals—as splits within the Lebanese government leave a vacuum in which they can pursue their violent activities.

Small groups like the Abdullah Azzam Brigades, which claimed responsibility for Tuesday's attack, are growing in strength, bolstered by millions of dollars from private Saudi donors, Lebanese and U.S. officials said. The funds go to Sunni Islamist militants seeking to help overthrow Syrian President Bashar al-Assad and strike at his Shiite allies, Iran and Hezbollah.

Some Lebanese leaders have turned a blind eye to the rise of such groups, worried that by confronting them, their support base would erode, Western officials said.

The violence, the first bomb attack on an embassy in Lebanon since the U.S. Embassy bombing in 1983, deepened fears of intensified violence spilling over from the war in Syria.

Admiral Dennis Blair, former Director of National Intelligence, talks to the WSJ's Sara Murray about the risk of an Al Qaeda comeback in Syria.

"We have entered a totally new phase, security-wise. The Syrian war is now here," said Nohad Machnouk, a member of parliament with the anti-Hezbollah Future Movement. "It will be more bloodshed…until there is a way forward in Syria."

The Abdullah Azzam Brigades described the attack as revenge for Iran and Hezbollah's support for the Assad regime. The group is known to operate in Lebanon but has never staged such a devastating attack.

On Wednesday, Hezbollah held military-style funerals for some of the victims, with yellow party flags draped over caskets.

The radicalization of Sunnis in Lebanon is most acute in the north, where an open war is being fought around Tripoli, the country's second-largest city.

Residents of Tripoli's suburbs suffer from poverty and illiteracy, and are openly hostile to a government they say does nothing to help them. Al Qaeda groups there have waged monthslong battles with Alawites, the offshoot of Shiite Islam to which the Syrian president belongs. Radical groups even ambushed the Lebanese security forces last month.

Concern about radical Lebanese leaders stepped up this year after al Qaeda's core leadership in Pakistan and Yemen sanctioned affiliates in northern Lebanon, according to intercepted communications between the parties, American and Lebanese officials said.

Rivalries between moderates of Lebanon's Sunni political class have allowed Lebanon's Sunni radicals to flourish over the last year. The clout of these politicians has waned in recent years, limiting their ability and will to support a crackdown on extremist groups, U.S. and Lebanese officials said.

In Tripoli's suburbs, men drink tea and play backgammon underneath black al Qaeda flags. Armed men who pledge allegiance to religious leaders patrol the streets in black jeeps with tinted windows and no license plates. Pop-up mosques have been created to collect arms and funding for rebels fighting in Syria or operations in Lebanon, changing locations weekly, from garages to abandoned buildings.

"We're fighting a war that's now out in the open up north," said an American official about growing extremism in Tripoli.

The religious leader of the group that claimed responsibility for Tuesday's attack, Lebanese Sunni Sheikh Sirajeddine Zuraiqat, is one of many radicals in Lebanon who have used political connections to keep their activities from being curbed.

Mr. Zuraiqat, thought to be in his mid-20s, only recently turned to radical religious ideology, said an acquaintance.

He used to run a cellphone shop in a central Beirut neighborhood.

This year, Mr. Zuraiqat was detained by Lebanese army intelligence agents on suspicion of ties to terrorism, said an official who works in the office of the Grand Mufti, the country's leading Sunni religious authority.

Mr. Zuraiqat's parents came to the mufti and asked for him to intercede on their son's behalf, said a person familiar with the negotiations. Soon after, Mr. Zuraiqat was released.

A spokesman for the mufti's office said authorities haven't proved their allegations against Mr. Zuraiqat. He added that the mufti would support the sheik's imprisonment if he was responsible for Tuesday's bombings.

A more prominent Sunni leader, Shadi Mawlawi, openly roams the streets of Tripoli's Qibbeh district, an area he said that the national army "wouldn't dare enter."

Mr. Mawlawi was arrested by Lebanese authorities last year, following a tip by American officials who suspected him of financing and arming al Qaeda-linked Syrian rebels.

The 27-year-old was charged with belonging to Jabhat al Nusra, an al Qaeda offshoot in Syria. He denied the charges. The U.S. Embassy declined to comment.

Mr. Mawlawi's arrest sparked deadly clashes in Tripoli that lasted 11 days, ending when he was released on a $333 bail—paid, he said, by Prime Minister Najib Mikati.

Mr. Mikati denied paying Mr. Mawlawi's bail. But upon his release, Mr. Mawlawi was driven in the luxury sedan of Lebanon's finance minister to Mr. Mikati's house in Tripoli, as fireworks were set off in his honor by supporters.

"I ate kanafi with Mikati, and it was delicious," Mr. Mawlawi said, referring to a famous Tripoli dessert.

Mr. Mikati acknowledged receiving Mr. Mawlawi, but said he showed up uninvited. "I didn't do myself, anything to release him…the judge took that decision," he said. "I didn't ask that he come to my home."

The case against Mr. Mawlawi was reopened this spring by Lebanon's Military Tribunal.

Mr. Mawlawi denied the charges of being a member of Jabhat al Nusra, an al Qaeda offshoot in Syria. But he boasted that, along with other prominent, government-supported sheiks, he has raised up to $1.5 million for Syrian jihadis and sent them "hundreds of arms" over the last two years.

"We are all victims in the north, Beirut does not care," he said, referring to the central government. "No one is addressing the plight of Sunni [Muslims].…Shadi Mawlawi is doing what all other players are doing in this game, to protect their own side."