SAN DIEGO (CBS 8) - Law enforcement agencies across the country are on alert, beefing up security ahead of the Fourth of July due to a possible terror threat. Here in San Diego, a warning went out to MCAS Miramar.

CBS News 8 obtained a letter that went out to those on MCAS Miramar, saying: "If you notice something that just doesn't look right, take the extra effort and report it to local authorities. Have a plan on what actions you and your family are going to take if you find yourself in the midst of a terrorist attack."

That warning comes as big cities like New York are increasing security. Earlier this week, CBS News Senior Security Contributor Michael Morell talked about the threat.

"These kinds of warnings go out routinely, but there's nothing particularly routine about this one. This one really resonates with me," said Morell.

He cited the increased number of people who have been arrested in the U.S. on terrorism charges. The other is the call to arms by terrorist groups during the Islamic holy month of Ramadan.

As for San Diego, CBS News 8 spoke with local FBI Special Agent Darrel Foxworth.

"Here in San Diego, we do not have any specific credible threat information at this time," says Foxworth.

Foxworth says the biggest threat is posed by lone wolves, people inspired by terrorist rhetoric who may act alone. He says people should go out and enjoy their Fourth of July, but be vigilant.

"If you do see something that seems unusual, something that's strange, something that causes you concern, contact law enforcement," says Foxworth.