It took about two days for the FBI to announce it was investigating the Dec. 2 attack that killed 14 in San Bernardino, California, as an act of terror.

Nearly five months after the killing of five military personnel in Chattanooga, Tennessee, authorities have carefully avoided using the same wording about the attack by a Kuwaiti-born gunman.

As a practical matter, labeling Chattanooga a terror attack would make its victims eligible for the Purple Heart, which if awarded would entitle their survivors to additional payments and benefits.

There's also a key difference between Chattanooga and San Bernardino in terms of how publicly authorities have announced investigative details and drawn conclusions, said former federal prosecutor David S. Weinstein: the ramped-up visibility of extremism after the deadly attacks in Paris blamed on adherents to the Islamic State extremist group.

"What has changed is U.S. and world perception about terrorists and how the U.S. is combating terrorism," he said. "That national pulse about terrorism is high."

At a news conference the day of the Chattanooga shooting, then-U.S. Attorney Bill Killian said the shooting was being investigated as an act of terrorism. Minutes later, he backtracked, saying the investigation would determine whether it was terrorism or some other crime.

The closest federal authorities have come since to calling Chattanooga a terror attack was Dec. 6, when President Barack Obama addressed the nation from the Oval Office after San Bernardino. The president noted that as the U.S. has improved in preventing large-scale assaults like those on Sept. 11, 2001, "terrorists turned to less complicated acts of violence like the mass shootings that are all too common in our society."

Obama then cited both Chattanooga and San Bernardino, as well as the 2009 shootings at Fort Hood, Texas, that killed 13 people and wounded 32 more. The Fort Hood attack, initially called an instance of workplace violence by many authorities, resulted in the murder conviction and death sentence of Nidal Hasan, a former U.S. Army major who said during his court-martial he believed he was defending Taliban leaders from American troops.

Joyce McCants, a spokeswoman for the FBI in Knoxville, Tennessee, which oversees Chattanooga, said Thursday that the bureau is likely to provide an updated statement this week about the July 16 killings by Muhammad Youssef Abdulazeez.

Abdulazeez, 24, was fatally shot by police after opening fire at a military recruiting center and then driving to a reserve center, where he killed four Marines and a sailor.

Within hours of the shooting, Attorney General Loretta Lynch issued a statement describing a "national security investigation" -- which immediately suggested the possibility of terrorism. No such statement came from the Justice Department on the night of the San Bernardino attacks as investigators worked to determine a motive.

Initially, authorities described Abdulazeez, a naturalized U.S. citizen born in Kuwait, as a homegrown violent extremist. Vice President Joe Biden said at a memorial for the Chattanooga dead that Abdulazeez was a "perverted jihadist" who may have been inspired to become "a single lone wolf to commit a savage act."

In the days after the San Bernardino massacre, the FBI labeled the married couple behind the attack terrorists who had been "radicalized" with Islamic extremist views.

The word "terrorism" is used by civilians and government officials alike to describe many killings that may have political or religious overtones, but terrorism is also a federal criminal charge. In essence, federal code defines terrorism as any one of a list of violent acts that is "calculated to influence or affect the conduct of government by intimidation or coercion, or to retaliate against government conduct."

The Associated Press reported previously that Abdulazeez had visited an uncle in Jordan before the Chattanooga shooting, but it's not clear if that visit prompted him to adopt extremist views.

Soon after the attack, investigators found writings from Abdulazeez that reference Anwar al-Awlaki, a U.S.-born cleric who encouraged and inspired attacks on the homeland and was killed in a U.S. drone strike in 2011.

Representatives of the family have said Abdulazeez was helping out with his uncle's small cellphone business in Jordan. The family has also described Abdulazeez as having problems that included depression and drug abuse. A representative for the family declined additional comment last week.

Ed Reinhold, special agent in charge of the FBI's Knoxville office, has said investigators believe Abdulazeez was acting on his own.

FBI Director James Comey told reporters during a visit last month to Nashville, Tennessee, that the bureau may never publicly reveal the results of its Chattanooga investigation.

"Sometimes the way we investigate requires us to keep information secret. That's a good thing. We don't want to smear people," Comey said.

Those killed were Navy Petty Officer 2nd Class Randall Smith and four Marines: Staff Sgt. David Wyatt, Sgt. Carson Holmquist, Gunnery Sgt. Thomas Sullivan and Lance Cpt. Squire "Skip" Wells.

U.S. Rep. Chuck Fleischmann, R-Tennessee, has introduced a non-binding resolution in Congress calling for Purple Hearts to be awarded in this case.

"These men proudly served their country, and several made the ultimate sacrifice to save others," Fleischmann said.