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By Robert Dembo, Miguel Almaguer and Alastair Jamieson

FBI agents searched a Tennessee apartment as investigators tried to determine whether at least two acquaintances of Chattanooga shooter Mohammad Abdulazeez knew of his intentions in the months leading up to the deadly gun attacks.

Officers converged on the Mountain Creek Apartments complex in Red Bank, north of Chattanooga, on Monday afternoon.

Abdulazeez, 24, opened fire at a military recruiting station and a Navy and Marine reserve center Thursday, killing four Marines and a sailor and wounding a local cop.

A family representative told NBC News that he was battling depression, had expressed some anti-American feelings and was on a three-day downward spiral before the attack — a period in which he smoked marijuana and went joyriding in the rented Mustang convertible that was later used in the attack.

The gunman argued with his parents when he last saw them on Tuesday night, the representative said. His parents knew their son was struggling but never thought he was a real threat.

Ed Reinhold, the FBI special agent in charge of the investigation, said: “We’ve been going all over this city talking to different friends and associates of our shooter. In order to make sure we have this investigation completely done we talk to everyone that we possibly can who may know or have been associated with the shooter.”

Joseph Christopher, a former high school friend of Abdulazeez, told NBC News he was surprised at the attack, and remembered the shooter as a religious teen who prayed regularly.

Bilal Sheikh, 25, said he saw his friend at the mosque two weekends ago, as they came to pray and as part of the services to celebrate Ramadan. "I'm in total shock, like everyone else," Sheikh told The Associated Press. "He was always the most cheerful guy. If you were having a bad day, he would brighten your day."

However, Abdulazeez was also due to appear in court later this month on a DUI charge and had been treated for depression since his teens.

The Associated Press contributed.