The July 16 shootings in Chattanooga, Tennessee, which left four Marines dead and one sailor wounded, are part of a string of attacks — some successful and others thwarted — against military recruiting stations in recent years.

"Recruiting offices have been kind of on the leading edge of targets simply because they are both ubiquitous and they're vulnerable," said Brian Michael Jenkins, senior adviser to the president of  Rand Corp.

The gunman in Chattanooga, identified as Mohammad Youssef Abdulazeez and also killed in the incident, targeted a joint-service recruiting station and a nearby Navy Operational Support Center, where the Marines were killed and the sailor was wounded.

Both al-Qaida and the Islamic State group have called for attacks on service members and their families.

Jenkins stressed that he has not seen any official confirmation about what the motivation was behind the  shootings, but they  could fall within the pattern of Islamic extremists targeting the U.S. military.

"These recruiting offices are everywhere," he said. "They're in shopping centers. They're all around the country. So if you think about attacking a military target, as opposed to driving to some military base where there will be armed guards at the gate; if you want a geographically convenient, readily accessible target that the shooter can portray as a military target, then recruiting stations fit the bill. So the attack, while shocking, is not surprising."

The U.S. should expect such attacks to continue as part of the overall war against al-Qaida and the Islamic State group, Jenkins said.

Officials from the Marine Corps could not be reached for comment by deadline regarding the identification of the Marines who were killed and the wounded sailor.

All Navy recruiting stations were open for business Friday, except the station in Chattanooga, which is still a crime scene, officials said. A Navy spokeswoman declined to comment on whether the service had stepped up security recruiting stations in the wake of the attack..

Military officials said they are not looking to heighten security at recruiting stations. An executive order spells out under what circumstances service members can be armed, but recruiting duty is not one of them.

As of now, U.S. Northern Command has no plans to raise the security levels any higher at U.S. military bases, officials said. NORTHCOM raised  the force protection level for military installations to "Bravo" in May, shortly after two gunmen were killed in Texas before they could attack a contest to depict the Prophet Muhammad as a cartoon.

Representatives from the Army and Air Force said current force protection procedures are adequate. Officials from the Navy and Marine Corps could not be reached for comment by deadline.

Other than the recruiting station targeted in Tennessee, all of the Army's recruiting centers remain open for "business as usual," said Brian Lepley, a spokesman for U.S. Army Recruiting Command.

Lepley said the Army trains recruiters annually on active shooting scenarios, force protection awareness and security measures.

The Army typically reviews policies in the aftermath of such incidents, Lepley said, but it's too soon to know if one might prompt changes. He suggested a fundamental overhaul to be unlikely.

"We can't have barricaded centers. We can't have places where we recruit young men and women that look like a fortress," Lepley said. "We have to have a connection to the American people."

He said no bullets went into the Army area of the recruiting center attacked in Chattanooga.

Two Air Force applicants and one Air Force recruiter were in the area at the time of the shooting, but escaped unharmed, showing the security procedures work, said Christa  D'Andrea, a spokeswoman for the Air Force Recruiting Service.

"The security measures we had in place, they followed them, as they were supposed to, and it worked for them," she said.

President Obama issued a statement saying  it appears the suspected gunman acted alone.

"We take all shootings very seriously," Obama said. "Obviously, when you have an attack on a U.S. military facility, then we have to make sure that we have all the information necessary to make an assessment in terms of how this attack took place, and what further precautions we can take in the future. And as we have more information, we'll let the public know."

There have been several previous attacks.

Abdulhakim Mujahid Muhammad is serving a life sentence in prison for killing one soldier and wounding another outside a recruiting station in Little Rock, Arkansas, in 2009.

In 2013, former Marine Yonathan Melaku was sentenced to 25 years in prison for firing at several military installations in 2010, including a Marine Corps recruiting station in Chantilly, Virginia, and a Coast Guard recruiting station in Woodbridge, Virginia.

Not all planned attacks are successful. Abu Khalid Abdul-Latif and Walli Mujahidh were arrested in 2011 for allegedly planning attacks on a recruiting station in Seattle. In 2012, Antonio Martinez, who had changed his name to Muhammad Hussain, was sentenced to 25 years in prison for planning an attack on a recruiting station in Catonsville, Maryland.

Kyle Jahner contributed to this story.