"Beirut city, itself, the downtown area, is very empty," reports Angus MacSwan, Reuters chief desk editor for the Middle East, of the usually "buzzing" city.
"Shops are closed, cafes are closed," he said on Monday. "The army has issued a very firm statement saying they were not going to let it get out of hand."
Marwan Charbel, the country's interior minister, announced early Tuesday that the car bomb used to kill al-Hassan was placed in a stolen Toyota RAV 4, according to a CNN update.
This could provide a critical lead in the investigation into the assassination.
An FBI team was heading to Libya Tuesday morning to assist with the investigation. The deputy secretary general of Hezbollah, Sheikh Naim Qassem, responded that this was an internal matter.
Qassem also said early Tuesday the assassination was designed to "incite internal strike" and create instability in the country.
Hezbollah, the militant political party and powerful ally of Syria, dominates Lebanon's government.
Learn more from this CNN blog.
State Department spokesman Mark Toner said Monday the department was waiting on the results of the FBI investigation before taking action.
"We're certainly concerned," Toner said in a briefing. "We've been clear for some time about the possibility of a possible spillover effect from the conflict in Syria. But what I can say, as the Secretary said in her call [with Lebanese Prime Minister Mikati] yesterday, is that we are prepared to offer assistance."
The international community has watched the violence with growing concern as the death toll continues to rise, while refugees from neighboring Syria flood into nearby countries.
More than 100,000 are in Lebanon as of Monday, according to a U.N. refugee agency report.
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Paul D. Shinkman is a national security reporter for U.S. News & World Report. You can follow him on Twitter or reach him at email@example.com