This Fourth of July weekend has spawned a flurry of strong warnings about a potential Lone Wolf attack as the federal authorities and national security experts say the United States is more susceptible now because of tactics chosen for recent terrorist strikes by the Islamic State.

Increased awareness has been common at national landmarks and large public events ahead of major holidays or significant anniversaries since Sept. 11, 2001. Officials cite an increased effort by the Islamic State to galvanize its sympathizers in the United States and elsewhere for the past two months and during this Ramadan season to carry out acts of violence on their own — so-called lone-wolf attacks. Those potential plots by individuals are harder for the authorities to detect and disrupt, senior American officials say, and have led the F.B.I. to put more Americans under investigation for suspected ties to terrorist groups than at any point since Sept. 11.

American officials acknowledge that there are no specific threats. The Islamic State, also known as ISIS or ISIL, has shown success at inspiring rather than actually directing well-planned attacks — and officials blamed the militant organization for inspiring last week’s bloody strikes in France and Tunisia.

Intelligence officials have seen at least one Twitter message of an Islamic State supporter citing the United States government’s warnings for this holiday weekend — apparently in boastful terms. While officials say they do not know of any specific threat, they also know they will be the first ones criticized by the public and members of Congress if there is an outbreak of violence.

In particular, American officials say, the Islamic State has told its followers to attack law enforcement and uniformed military personnel, many of whom are prominent in public settings on July 4.

Independent analysts who follow jihadist statements on the Internet said they have not detected any significant increase in online comments — what the authorities called “chatter” — but acknowledged that lone attackers inspired by ISIS leaders and propaganda could go undetected.