Representative Alexandria Ocasio-Cortez accused President Donald Trump of sending white supremacists a message.
New Zealand Prime Minister Jacinda Ardern pledged to do everything she could to stifle the suspected gunman's desire for publicity.
Trump infamously called "for a total and complete shutdown of Muslims entering the United States" before taking office.
A survivor of the Linwood Mosque attack told a New Zealand news outlet that the shooter could not find the main door and instead shot at a window from outside.
The country is set to change its gun laws within days of an attack that claimed the lives of 50 people.
"There was a want to receive the dead immediately and start the grieving process...but they realize that this is not like any other incident," said one family member.
"Once you accept that these things can be harmful in the wrong hands, the trade-off is a small inconvenience," one gun owner said.
"We would like to remind people that it is an offense to distribute or possess an objectionable publication," New Zealand police warned in a statement.
Right-wing radio host Jesse Lee Peterson expressed concern that repressed white Americans may be incited to carry out a New Zealand–like attack in the U.S.
Australian Senator Fraser Anning is said to have hit the teenager after he egged him.
"But at the end of the day, these types of incidents are the result or the fault of the person behind the gun," Alice Stewart said.
"There is real data and information currently, right now on the rise of white supremacy right here in this United States of America," Representative Rashida Tlaib pointed out.
"You are my friends. I will keep watch while you pray," Andrew Graystone's sign read.
"When Putin interferes in the election, he's impotent," a New York Times opinion writer said. "The question is, why?"
"New Zealand is a sickening tragedy," Coulter tweeted. "When will this insane terrorism end."
American supporters of Donald Trump littered social media with posts condemning accusations tying the president's anti-Muslim, anti-immigrant rhetoric to Friday's shooting at two New Zealand mosques.
As people across the globe mourn, questions have been raised about whether the attack could have been prevented with better intelligence sharing and online surveillance.
"This president has given license to white supremacy in America that has spread across the world," said Tom Steyer, founder of the Need to Impeach campaign.
The alleged shooter called President Donald Trump "a symbol of renewed white identity and common purpose."
Progressive Twitter users lamented the massacre and used it as an example of the consequences of weak laws on gun ownership.
"I was on the ground faking it," Mustafa Boztas said.
"As a New Zealander, I can't imagine a country that is less likely to ignore this and do nothing as the United States has done after Columbine, Sandy Hook and all the others," said gun control expert Philip Alpers, a researcher at the University of Sydney and founding director of GunPolicy.org, which tracks gun laws around the world.
About a half hour after the post, the mass shootings were reported at two mosques in Christchurch, New Zealand.
The conservative political commentator was cited as the person who "radicalized" the alleged gunman the most.
"I feel absolutely sickened having my name uttered by this person," PewDiePie tweeted following the deadly shooting.