Mohammed Malik told CNN's "Erin Burnett OutFront" that he didn't think Mateen fit the profile of a radical but he was concerned about comments Mateen made to him.
"He told me they were powerful," Malik said. "... those raised the red flags for me and prompted me to speak with the FBI."
The fact that Mateen was a family man made Malik think he wasn't a radical but because another man from their mosque had been a suicide bomber in Syria, Malik thought he should say something.
FBI Director James Comey seemed to reference Malik when he said on June 13: "In the course of that investigation, one witness told us, when asked, "Do you know anybody else who might be radicalizing," that he had once been concerned about the killer because the killer had mentioned al-Awlaki videos. The witness had concluded that he later got married, and had a child, and got a job as a security guard, and so he was no longer concerned about him.
Comey said the FBI spoke with Mateen
about his connection to the suicide bomber.
In his Post piece, Malik said he was writing because presumptive Republican presidential nominee Donald Trump had said no Muslim did anything to stop Mateen.
Not so, Malik said.
"We do speak up. The only thing is we don't want accolades for what we do," he said. "The only reason I am coming out is because of the hatred that I've seen spewed because of this incident."
He said many Muslims do the right thing, but it isn't something that becomes public.
Mateen and Malik were friends who saw each other at the mosque and spoke by phone a few times a year, according to his op-ed. He said he saw nothing dark about Mateen.
And he told CNN that Mateen never really said anything being gay or being homophobic.
Also Tuesday, the killer's father, Seddique Mir Mateen, said that his son had been interred Sunday in an Islamic burial. The father didn't give any details about the burial.
Attorney general visits
U.S. Attorney General Loretta Lynch traveled Tuesday to Florida to visit with survivors of the Orlando nightclub shooting and check up on the investigation into shooter Mateen's rampage.
She brought $1 million in emergency funding to help Florida, Orange County and Orlando pay for overtime and other investigative costs, and more praise for a community that has famously united in the face of tragedy.
Lynch expressed great pride in the community and its unity "in defiance of terror and in defense of our most cherished ideals."
"We stand with you today as we grieve together and long after the cameras are gone, we will continue to stand with you as we grow together in commitment, solidarity and in equality," she said.
Lynch's trip comes a day after authorities released details of Mateen's communications with police during the June 12 mass shooting at Pulse nightclub, where 49 people died and more than 50 others were wounded.
On Tuesday, law enforcement officials told CNN that Mateen had visited the club
earlier in the night before his attack, possibly to assess the club's security.
Meanwhile, police reopened streets near Pulse and wound down their investigation at the crime scene, leaving behind, in the words of the police department's Twitter account, "profound sadness."
Lynch declined to answer questions about the investigation and whether authorities are looking to charge anyone else in connection with the case.
Full transcript released
Lynch and authorities took some heat Monday after issuing a redacted transcript of Mateen's call to 911 after the first burst of shooting in which he claimed responsibility for the attack and pledged allegiance to the ISIS terror group and its leader, Abu Bakr al-Baghdadi.
House Speaker Paul Ryan called the decision to omit the names of ISIS and its leader "preposterous."
Later, the FBI reversed course and issued a full transcript
of Mateen's 50-second 911 call.
"My name is I pledge of allegiance to Abu Bakr al-Baghdadi of the Islamic State," the revised transcript of the call reads.
The decision to leave the names out started with FBI agents in the Orlando field office who were following Comey, who generally refuses to use the names of terrorists, CNN's Evan Perez reported, citing law enforcement officials.
Officials at FBI headquarters and the Justice Department signed off on the decision, the law enforcement officials said.
At a news conference Monday, Orlando-based FBI Special Agent Ron Hopper defended the move.
"We're not going to propagate their rhetoric, their violent rhetoric," he said.
Speaking to reporters Tuesday, she said she would be open to releasing the audio of Mateen's 911 call and possibly other details from the investigation.
"We are looking to be as transparent as possible and provide as much information as possible," she said.
Police actions defended
In the aftermath of the shooting, some survivors had spoken out shortly after the events unfolded, criticizing a three-hour standoff that followed the early burst of gunfire.
"Why couldn't we have taken care of this much earlier?" New York television station WNYW
quoted attack survivor Jeannette McCoy as saying two days afterward. "There were people in there bleeding to death. A part of me puts a sense of blame on (police). I'm sorry, but I was there. They could have done something."
But Monday, Orlando police Chief John Mina reiterated earlier statements he had made that circumstances changed when officers ran to the gunshots and fired on Mateen, driving him into a club bathroom with hostages.
At that moment, Mina said, what had been an active shooter situation requiring officers to engage the gunman turned into a hostage situation that demanded negotiation and planning.
"The timeline released based upon radio communications clearly shows our officers were within the club within minutes and engaged the suspect in gunfire," Mina said Monday. "And that's important because that engagement and that initial entry caused him to retreat, stop shooting and barricade himself in the bathroom with hostages."
And, he said, police were in and out of the club rescuing victims throughout the three hours between when police gunfire drove Mateen into the bathroom and the final encounter with the gunman after a SWAT team blew a hole in the wall and stormed in.
At the same time, crisis negotiators were trying to talk to Mateen to end the situation without further bloodshed, and the SWAT team was setting up for its entry, Mina said.
There was no gunfire during that time, he said.
When Mateen threatened to put explosive vests on hostages -- vests authorities later found he did not have -- commanders gave the order to go in, Mina said.
"So I'm extremely proud of the heroic actions of our officers, and I am very confident they saved many, many, many lives that night," he said.