The FBI is seeking to interview anyone with whom Abdulazeez may have had contact in the 48 to 72 hours before Thursday's shootings, the official said.
As the country mourns the deaths of the service members, investigators in both the United States and Jordan are trying to learn what prompted the attack.
Abdulazeez first shot up a military recruiting center at a Chattanooga strip mall, then drove to a local Navy operations support center and launched another attack, killing four Marines and a sailor. The gunman died in a gunfight with law enforcement.
Abdulazeez told a friend that ISIS
was "doing wrong" and "it was a stupid group and it was completely against Islam," the friend told CNN on Monday.
The friend, James Petty, also said that Abdulazeez taught him how to shoot an AR-15 assault rifle and that the two would practice in the woods.
Writings indicate anti-U.S. sentiments
The revelations about the gunman come as sources told CNN that writings uncovered by investigators indicated Abdulazeez was displeased with the U.S. government, particularly its war on terror.
The writings are not thought to be recent -- some are more than a year old, predating a much-publicized trip to Jordan -- and should not be considered a diary of any sort, according to a person familiar with the family's interviews with investigators and a law enforcement official briefed on the investigation.
Abdulazeez's uncle, Ibrahim Asaad Haj Ali, has been held by Jordanian authorities since Friday and has not been allowed access to legal counsel, his attorney told CNN Arabic.
Abdulazeez stayed at his uncle's home in Jordan in 2014 and worked at the company the uncle runs, lawyer Abdul Qader al Khatieb said. It's not known if any charges have been filed against the uncle.
FBI officials already stationed in the region are working with Jordanian counterparts on any information that could be helpful to the investigation, according to a law enforcement official briefed on the investigation.
The writings also include other anti-U.S. sentiments and are consistent with someone who is having suicidal thoughts, the sources said.
Abdulazeez suffered from depression and "was not the son we knew and loved," his family said in a statement over the weekend. "We extend our deepest sympathies and condolences to the families of the honorable service members and police officers who were victims of the shooting our son committed on Thursday." A police officer was among the wounded.
The family has also told investigators the 24-year-old had been abusing drugs for some time, according to a source familiar with the family's interviews with investigators. The drugs reportedly included "party drugs" and marijuana.
Petty, Abdulazeez's friend, said his buddy had a drug problem and used marijuana heavily, so much so that his parents were constantly calling to check on his whereabouts.
Abdulazeez's family sent him to Jordan last year to get him away from Chattanooga friends who they said were bad influences on him, the relatives told investigators. According to a family representative who asked for his name not to be used, the parents sent their son to Jordan to stay with a family in the hopes of getting him away from his life of depression and drug use.
Some relatives and friends told investigators they detected changes in his behavior after he returned from Jordan last year, a law enforcement official said.
His victims came from across the country,
but united in Chattanooga, brought together by their service in the military.
Marine Gunnery Sgt. Thomas Sullivan of Massachusetts was a two-time Purple Heart recipient. He served three tours of duty.
Lance Cpl. Squire K. Wells, better known as "Skip," graduated from high school three years ago. The Georgia native joined the Marines in 2014 and had already earned a National Defense Service Medal.
Marines Staff Sgt. David Wyatt of Arkansas served two tours of duty in Iraq and one in Afghanistan. His wife posted about his death on Facebook. "He was such a great husband and father," one commenter wrote.
Marines Sgt. Carson Holmquist was also a husband and father and served two tours of duty in Afghanistan. Upon one of his homecomings, his family held up a sign that read, "We've waited 244 days for this moment. Welcome home Sgt. Holmquist."
Navy Petty Officer Randall Smith of Ohio was a logistics specialist. He saw the gunman in last week's attack and warned people around him, family members said, but was unable to get away.
In the response to the shootings, some governors are increasing security measures for National Guard recruiters and military facilities. Several have called for arming National Guard members