Charles Leffingwell, a resident of Washington, DC, described the message as "devastating."
"It is traumatic for all Muslims I know – everyone worries about it," he said.
When he visited prayers in his mosque on Friday, Leffingwell prepared for the worst.
"I thought something could happen on some level and expected us to hear shots while we prayed," he said.
Leffingwell Scenario I thought it was similar to the shootings in Christchurch, which the authorities called a terrorist attack. The gunman entered the mosques for Muslims on the holiest day of the week and unleashed a rifle fire.
The alleged shooter livestream video of the attack. In a manifesto published shortly before the attack on the Internet, he described himself as an Australian-born white man. The document advocated anti-immigrant and anti-Muslim rhetoric and introduced white nationalists who inspired it.
Manar Khalil of Staten Island, New York, said the fatal incident intensified her fear of bigoted attacks, especially as a woman whose headscarf makes her "visibly Muslim"
"I can not stop thinking about it everywhere and could happen anytime, "she said. "I am afraid for my family and other Muslims who have become the targets of hatred and pointless murders."
"Today my husband went to the Friday prayers and I was thinking about it all the time," she added.
In Vancouver, Washington, Rabia Haq said community members were also concerned about attending prayer services.
"[My sister and friends] spoke in group chat – some of them do not want to go," said Haq. "You are scared."
In the end, however, they decided to leave.
American authorities on alert
The authorities said on Friday that there are no known mosque threats in the United States. Nevertheless, police across the country have increased their presence in Islamic worship and other religious sanctuaries.
New York Governor Andrew Cuomo and New York City Mayor Bill de Blasio called for increased security in mosques and places of worship. NYPD officers were in front of the Islamic Cultural Center in New York on Friday afternoon.
Police officers in Atlanta and Chicago also released statements indicating that they are on high alert for suspicious activity near mosques. The state police in Massachusetts said they would increase the frequency of patrolling mosques and other religious institutions.
American Muslims and local authorities have cause for concern as anti-Muslim prejudice and hate crimes increase.
Despite increased security, American Muslims still expressed fear.
Leffingwell said prayer organizers after the Friday worship service Wors told Hipper that a nearby church would provide security in the future. "I do not feel safer," he said.
Haq said her Vancouver mosque always has a police officer on duty, but friends and family "still do not feel safe."
The Mosque a few streets From Khalil's home in Staten Island, he even announced that he would start training the active shooter and doing the exercises in response to the New Zealand attacks. "Who would have thought that a place of worship and rest should be prepared for such a thing?" She said.
American Muslims need more security to feel safe, said Abdelgadir Elmadani, resident of Ashburn, Virginia.
"It's just a temporary measure of a bigger problem," said Elmadani. "If we really want to deal with the problem, we can not just look at the symptoms, we have to tackle the cause of the problems."
Many American Muslims say that hostile rhetoric is blamed on political leaders and media experts.
"This issue has been given a permit by an entire government and political culture that keeps an eye on the toxicity of this system," said Abbas Barzegar, director of research and advocacy at CAIR.  "It's very easy to transcribe tweets or messages from a specific political leader, but we have to recognize that rhetoric is now anchored in politics." He said that under these policies, President Donald Trump's travel ban was directed against several Muslim-majority countries.
Despite their fears, American Muslims who had spoken with CNN continued to say determined worship in mosques and fight against hatred in all its forms.
"It's scary, but it will not stop us from getting into the Masjid," said Danish Shaikh, who was in Atlanta on Friday at Al-Farooq Masjid, Georgia. "This is a home for us, we went to the Masjid on Friday."
Khadijah Khan, another worshiper at Al-Farooq Masjid, said she was in pain when she wanted to know what was happening in Christchurch, but she did not want that their decision was affected by the mosque.
"As Muslim Americans, black Americans, all of us who are minorities, we are all standing," she said. "We can not be afraid and we can not go back in. Once you've retired, forget about your identity and that's what you want."
Elmadani also said the attack did not change his plans to attend prayer services. "A coward attack should not intimidate or force us to change our way of life," he said. "That's what terrorists want, and we should not let them have that satisfaction."
It was a feeling that was repeated by Muslim leaders in the United States. They said that the fears of the community were legitimate, but urged them to remain courageous and to worship freely.
Nihad Awad, CAIR's National Executive Director, also encouraged American Muslims to attend the prayer service on Friday.
"Do not be afraid and do not leave your mosques – not today" Not always, "he said at a press conference." They want you to be scared. They should not be afraid. "
Asim Malik of Austin, Texas, said that although he does not go to the mosque on a regular basis, he was attacked by Christchurch's attack on Friday.
" Islamophobia is as real as anti-Black. " Racism, anti-Semitism and the like, "he said.
" I do not think hiding or staying under the radar is the answer. When white supremacists attack one of our communities, they attack us all. We should fight for and protect the rights of others to life, freedom and the pursuit of happiness.
Nadeem Muaddi of CNN contributed to this report.