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Members of the small Muslim community in Fort Pierce, Fla., where Omar Mateen lived, strongly condemned his actions and said they feared the sort of backlash that some Muslim people faced after the Sept. 11, 2001, attacks.

Dr. Adel Nefzi, a chemistry professor at a local college and a board member of the Islamic center where Mr. Mateen prayed about three times a week, said a man had chanted “bad words against Muslims” in front of the center shortly after the attack, until the police came and escorted him away.

“Like all Americans, we are under shock, we cant believe what happened,” he said. “We are very sorry to the familes and our sympathies are with the families. We have encouraged our community to go give blood and help save lives. “

Dr. Nefzi described his interactions with Mr. Mateen as not substantial enough to have a character judgement — just “10 or 15 minutes” every time Mr. Mateen came for prayer, sometimes with his young son.

The concerns about a backlash were not limited to Mr. Mateen’s immediate community. Maged Metwally, a leader of an Islamic center in Stuart, Fla., about 30 minutes from where Mr. Mateen grew up, said tensions were high.

“It took us four to five months to resume our normal lives after 9/11,” Mr. Metwally said. “We are fearing worse this time — but obviously it depends on what the F.B.I. comes up with.”