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The gun believed to have been used by the Fort Lauderdale, Florida, airport shooter to kill five people and wound six more is the same weapon that was once taken away from him because of his mental state, sources told NBC News on Sunday.

Former Army reservist Esteban Santiago, 26, walked into an Alaska FBI office with his infant child on Nov. 7 "to report that his mind was being controlled by a U.S. intelligence agency," Special Agent in Charge Marlin Ritzman told reporters Saturday.

Agents then called Anchorage police, who retrieved a gun from his car outside.

"Santiago was having terroristic thoughts and believed he was being influenced by ISIS," Anchorage Police Chief Chris Tolley said in a news conference Saturday.

Esteban Ruiz Santiago, 26, in a police booking photo. Broward County Sheriff's Office / AP

The FBI confirmed that Santiago's child was taken into the "constant custody and care of the FBI" until Santiago's girlfriend, the child's mother, could pick him up.

His brother Bryan Santiago told NBC News partner Telemundo in Puerto Rico that Esteban was taken by police to a mental health evaluation but was released after only four days.

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Police returned the handgun to him one month later, officials said.

On Sunday, federal law enforcement sources told NBC News that they believe that the gun authorities seized was the same weapon he allegedly used in Friday's shootings, a Walther 9mm pistol.

Ritzman said it's normal for people to deliver walk-in complaints at FBI offices across the country. He added that nothing in Santiago's actions or history indicated a need to do anything more than to take him to the mental health facility.

"There have been concerns raised about why Mr. Santiago was not placed on a no-fly list. I want to be clear, during our initial investigation we found no ties to terrorism," Ritzman said. "He broke no laws when he came into our office making disjointed comments about mind control."

In January 2016, however, Santiago was arrested and charged with assault and criminal mischief after a violent argument with his girlfriend in Anchorage. She told investigators that he broke down the door, choked her and hit her on the side of the head.

Nevertheless, on Nov. 17, Anchorage police sent Santiago a letter inviting him to pick up his gun from evidence lockup, Tolley said. The former soldier went by the police station on Nov. 30 to pick up his weapon.

Related: Airport Shooting Suspect Esteban Santiago Sought Help for Mental Problems: Brother

But law enforcement officials said Santiago was unable to leave with his weapon that day, as police felt compelled to discuss the issue with the local FBI office.

Police then made an appointment with Santiago for Dec. 8 to pick up his weapon.

The U.S. attorney for Alaska, Karen Loeffler, said federal law requires that someone be "adjudicated mentally ill" for him or her to be prohibited from owning a handgun, which she said is difficult.

Surveillance video shows Esteban Santiago as he fires his first shots in the baggage claim area of Florida's Fort Lauderdale/Hollywood International Airport on Friday. TMZ

"This is not somebody that would have been prohibited [from having a gun] based on the information that they had," she said. "Law enforcement acted within the laws that they had."

In interviews with the gunman, law enforcement officials learned that Santiago had planned the horrific attack. He traveled from Anchorage to Fort Lauderdale with just the handgun and two magazines legally in his checked baggage, which he took to the bathroom to assemble.

In a chilling video obtained by TMZ, Santiago is seen walking casually in baggage claim with the weapon tucked in his waistband. He then pulls it out and begins shooting.

Related: Fort Lauderdale Shooting: Three Victims Were Headed to Vacations

Santiago told law enforcement that he believes he fired 15 rounds before being arrested. Five people were killed, and six others were wounded.

The U.S. attorney's office for southern Florida has charged him with performing an act of violence against a person at an airport serving international civil aviation and two other counts. If he's convicted, Santiago could face the death penalty.