WASHINGTON: A Muslim American of Pakistani descent says he had informed the Federal Bureau of Investigation (FBI) about Omar Mateen, who killed 49 people in a Florida nightclub, after learning about his radical thoughts in 2014, and criticised Republican presidential candidate Donald Trump for accusing Muslims of not sharing such information with authorities.

"Trump is wrong that we [Muslims] don’t speak up when we’re able... I know this firsthand: I was the one who told the FBI about Omar Mateen," Mohammed A. Malik wrote in an op-ed in the The Washington Post on Monday.

Read: Trump says US should consider profiling Muslims

Malik said before opening up about Mateen, he told the FBI about Moner Mohammad Abu-Salha, a Muslim boy from his neighbourhood in Florida who became the first American-born suicide bomber when he attacked a government office in Syria in 2014.

Being a patriotic American, Malik said he offered to speak up about Moner after reports that American officials were looking for people to give them some background on the bomber.

“I am an American Muslim, and I wanted to do my part. I didn't want more innocent people to die.”

Following this, Malik, who came to the US from Pakistan at the age of six, said he met other members of the Islamic community, including Omar Mateen, to talk about the bomber, Moner. Both Malik and Mateen attended the same mosque as Moner in Fort Pierce, Florida.

"That’s when Omar told me he had been watching videos of (Anwar al) Awlaki, too," the Yemen-based Imam whose lectures had allegedly radicalised Moner.

Malik said it raised red flags for him when Mateen told him that the video lectures were "very powerful".

He then contacted the FBI again to let them know that Mateen had been watching Awlaki's tapes. The FBI looked into him and closed the file, however, after apparently finding nothing against him.

The author said Mateen's gruesome massacre of 49 people left him and the community "in shock".

"If you don’t agree with someone, you don’t have the right to kill them," he wrote. Malik's last conversation with Mateen was by phone in May when the latter called to tell him about his vacation in Orlando.

Malik lambasted Trump for his "tragic, ugly and wrong" assertions that Muslims hide information about radical elements in their community from authorities.

"When they see trouble they have to report it, they are not reporting it, they are absolutely not reporting it and that's a big problem," Trump had said in an interview broadcast in March this year.

Malik said he alerted the FBI about Mateen "because my community, and Muslims generally, have nothing to hide".

According to an editor's note posted under the op-ed, a US federal law enforcement official confirmed Malik's cooperation [about Mateen and Moner] with authorities to The Washington Post.