In a weekly bulletin sent to law enforcement officials across the country, the FBI repeated recent warnings the United States remained a top al Qaeda target, but said it had no specific information regarding a July 4 attack.

"Al Qaeda's targets generally have been of symbolic significance as well as directed at inflicting mass casualties with the timing of attacks driven more by operational considerations than specific timeframes," said the bulletin, entitled "Fourth of July Threat Environment."

"In recent overseas attacks, terrorists have employed small-arms equipped assault teams, large vehicle borne explosive devices, and suicide bombers," the bulletin said. "These or similar tactics could be used in the United States."

U.S. officials have said since April they were worried al Qaeda might take advantage of holidays or special events over the summer to attack the United States or U.S. interests.

The bulletin gave a number of possible infrastructure targets that included gas refineries and pipelines, subways and trains, financial institutions, civil aviation and nuclear power plants.

Noting suicide bombers had been used overseas to attack "soft targets" like restaurants or nightspots, the bulletin gave officers a list of visible indicators that someone might be a suicide bomber.

They include people wearing loose-fitting clothes or jackets even in hot weather, exposed wires under clothing, strange chemical odors or sweating and mumbling of prayers. It said suicide bombers may try to disguise themselves in military or police uniforms or pose as pregnant women.

Earlier this week, a top Homeland Security official said security would be tightened for the July 4 holiday weekend but that there was no intelligence of a possible attack that would justify raising the terror alert level.