Alan Godlas, a professor of religion at the University of Georgia who teaches about Islam, said terrorists who struck Western targets last year during Ramadan must have had other considerations.

"If anything, Ramadan would have been a factor in their not attacking because it is a holy month," he said. "But, as you know, with these militant extremists, the ordinary considerations of the religion seem to go by the wayside and their own political and military objectives are foremost in their minds."

"It's not really driven by significant dates or religious anniversaries," said Magnus Ranstorp, director of the Center for the Study of Terrorism and Political Violence at St. Andrews University in Scotland. "They have their own time schedule for attack. It's driven by opportunity. It's driven by logistical capability, and what they desire to do."

Last year, the United States was put on "orange" alert, the second highest, for most of February, in part because of threats related to the Muslim religious period of Hajj. At the time, federal officials said specific, corroborated intelligence pointed to an attack around the end of the religious holiday. The alert level was lowered once the holiday passed. It was not raised during Ramadan last year.

Several terrorist attacks occurred during Ramadan last year in Iraq and other locations, including:

— A suicide bombing at the Italian military command post in Iraq that killed 19 Italians and 13 Iraqis.

— Deadly vehicle bombs exploding at two synagogues, the British consulate and outside a British bank in Istanbul, Turkey.

— A suicide attack on an affluent, guarded residential compound in Riyadh, Saudi Arabia, that killed 17 and wounded 122, with many children among the victims.

— A car bombing outside the headquarters of the International Committee of the Red Cross in Baghdad that killed 12 people, including two Red Cross staff members, and injured 22 others. The attack has had a lasting impact. The humanitarian organization shut its office and scaled back operations throughout the country.

"We don't have a visible, recognizable ICRC address anymore in Baghdad and Basra," said Antonella Notari, a spokeswoman in Geneva. "We still have people there. They work extremely discreetly. We don't give much information on where they are and how many they are."

Ramadan is the ninth month of the Islamic year. The Muslim calendar follows a lunar cycle, with months beginning right after the new moon. Due to geographical distances, different countries can begin Ramadan one day ahead or later than the time decreed by senior religious councils in Saudi Arabia, the birthplace of Islam. The precise start of the holiday also varies slightly among Islamic sects, with the largest, the Sunnis, beginning on Friday.