SAN'A, Yemen -- Authorities foiled an attempt by suicide bombers to blow up two oil installations with explosives-laden cars Friday, days after al-Qaida threatened to strike facilities in the Persian Gulf. The four attackers and a guard were killed.

The violence coincided with an election campaign in which President Ali Abdullah Saleh, facing his first real challenge since he became head of state in 1978, has been reaching out to the country's most strident Islamic movement.

No group claimed responsibility for Friday's attacks. But Prime Minister Abdul-Kader Bajammal said "investigators have uncovered all the details related to the two operations and the identity of the perpetrators," according to the official news agency. He did not elaborate, but said work at the oil installations was proceeding normally.

There have been fears of an al-Qaida attack since 23 members of the terror network tunneled out of a Yemeni jail in February with help from prison guards. Fourteen prisoners remain at large.

Al-Qaida has an active presence in Yemen, the ancestral home of Osama bin Laden. Al-Qaida has been blamed for two attacks on ships off Yemen -- the bombing of the USS Cole that killed 17 American sailors in 2000 and the attack on a French oil tanker that killed one person two years later.

After the Cole bombing and the Sept. 11 attacks in the United States, the West began pressuring Yemen to join the war against terror. Saleh launched several crackdowns against extremists, winning praise from the United States.

Analysts say Friday's violence has al-Qaida's fingerprints, especially since it came after a videotape aired Monday of the network's No. 2, Ayman al-Zawahri, threatening attacks on the Gulf and on facilities he blamed for stealing Muslim oil.

The four bombers struck in two groups during shift changes at the facilities.

In the first attack, two suicide bombers drove "at great speed" toward the Dubba Port at 5:15 a.m. in an attempt to blow up storage tanks filled with "a huge amount of oil," the Interior Ministry said in a statement.

The driver of the first car wore a uniform similar to those of staff at the facility, and the second driver was in a military uniform, the statement said.

It said guards "managed to blow up the rigged cars before they reached their targets."

A security guard was killed while "remains of the two terrorist attackers were strewn all over the place," the statement said. Shrapnel from the cars started a small fire in one storage tank, but it was quickly put out, it said.

At 5:50 a.m, security guards at a refinery in Mareb blew up two white cars loaded with explosives.

"They were driven by other suicide bomber terrorists who tried to break into (the facility)," the Interior Ministry statement said.

While one vehicle was stopped outside, the other got through the gates and sped 50 yards into the compound, security officials said.

Guards opened fire and detonated it about 100 yards from pipelines containing more than 15,000 cubic feet of gas as well as a control room for pipelines bringing in crude oil, the officials said, speaking on condition of anonymity because they were not authorized to give the information to the media.

The two attackers were killed and no one else was hurt in the Mareb attack, it said.

The Interior Ministry said violence and threats would not block Yemen's "persistent efforts to fight terrorism and the terrorist elements of darkness." It said an inquiry was under way to determine the identity of the "terrorist elements" behind the attacks.

Al-Qaida has threatened repeatedly to hit oil infrastructure, but its only major attempt was in February on Saudi Arabia's Abqaiq oil processing plant, the largest such facility in the world. In that case, bombers in explosives-laden cars were stopped at the gates in a gunbattle with guards.

Researcher Abdul-Bari al-Taher said for the anti-terrorism campaign to succeed, Yemen needs to deal more firmly with the militants.

For instance, a senior al-Qaida operative was sentenced in May to only three years and one month in prison for involvement in the attack on the French tanker. In July, 19 alleged al-Qaida members were acquitted of plotting to assassinate Westerners, and the judge exonerated some of fighting U.S. troops in Iraq.

The ruling party's rapprochement with the hard-line Salafi movement will make extremists bolder, said al-Taher, a specialist in Islamic groups with the Yemeni Center for Studies and Research, an independent research institution.

The ruling General People's Congress Party turned to the Salafis after losing the support of the powerful Islah Party ahead of Wednesday's presidential election.

Senior Salafi leader Abul-Hasan al-Maribi appeared at an election rally in Mareb at which President Saleh spoke. Al-Maribi denounced the election, saying Saleh should not be challenged as ruler -- a common belief among some extremists.

"Playing the religious card is dangerous and the government should stop doing it," al-Taher said.

The regime's ties to Islamists date to the 1980s, and they were strengthened during the 1994 civil war when Islamic fighters -- many of whom had fought the Soviets in Afghanistan -- sided with Saleh's northern government against secessionists in the secular south.

After the war, the Muslim groups demanded rewards from Saleh's government. It obliged, appointing militants or their supporters to key positions in the army, police and administration.

Saleh faces four opponents Wednesday. The most serious challenger is Faisal bin Shamlan, who is backed by five opposition parties and has spoken out against al-Qaida and corruption.

AP correspondent Ahmed al-Haj in San'a contributed to this report.