WASHINGTON - There are no specific or credible security threats against the United States known to law enforcement heading into the Fourth of July weekend, the Federal Bureau of Investigation said on Thursday."However, law enforcement and members of the public must remain vigilant. ... The FBI asks members of the public to maintain awareness of their surroundings and to report any suspicious activity to law enforcement," an FBI spokeswoman said in a statement to Reuters.An internal briefing document shared with government officials and seen by Reuters called for the Department of Homeland Security to be on alert following recent attacks, such as the mass shooting in Orlando, Florida, on June 12, and calls from Islamic State supporters for attacks in the Western Hemisphere.The absence of known threats does not ensure safety. There were no threats known before the Orlando massacre, in which a gunman who pledged loyalty to Islamic State killed 49 people and wounded 53 more in the deadliest mass shooting in modern UShistory.The document said mass transit systems are particularly vulnerable to attack. It also said an attack over the US Independence Day holiday would garner media attention that would further Islamic State's propaganda efforts.There was no mention in the FBI's statement or in the document of the gun and bomb attack that killed 44 people at Istanbul's airport on Tuesday.