After New Zealand massacre, US authorities search for copycats and links to America
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WATCH: A gunman, dressed in tactical gear, appeared to livestream video of the shooting on social media, according to New Zealand police.

U.S. authorities and international partners were scrubbing their databases on Friday to determine whether the man now in custody in New Zealand for allegedly launching the deadliest mass shooting in New Zealand's history has any links to their own countries.

Police in New Zealand say the alleged gunman, described as being in his late 20s, opened fire inside a mosque in Christchurch and that a shooting at a different mosque killed a total of at least 49 worshipers gathered for Friday prayers. Authorities say a lengthy online document, posted after the shooting and linked to the alleged gunman's video of the shooting, praised other mass shooters, advocated white supremacy and detailed hatred for immigrants.

In the document, the author said he wanted to create a Second Amendment-inspired civil war inside the United States that would further divide Americans along racial and political lines.

So far, however, American authorities have not found any evidence that the alleged gunman traveled to the United States in recent years – if at all – according to sources familiar with the matter.

A woman arrives for service at the Islamic Cultural Center of New York under increased police security following the shooting in New Zealand, March 15, 2019, in New York.

Meanwhile, federal, state and local law enforcement agencies across the U.S. are looking to bolster security at mosques in their regions.

"There is no specific or credible threat to our homeland, but all Americans should remain vigilant so we can prevent such violent hate," Homeland Security Secretary Kirstjen Nielsen posted to Twitter on Friday.

She was unequivocal in her description of the New Zealand attack: "This was an act of terror."

A police vehicle sits outside of the Islamic Center of Washington in Washington, D.C., following the mosque attacks in New Zealand March 15, 2019.

In Washington, D.C., on Friday, police said they were providing "an increased presence at mosques in the nation’s capital," and city officials were contacting religious leaders in communities there.

In New York, the city’s police department announced that it had also deployed "extra NYPD officers" to mosques throughout the city.

"The NYPD is committed to the safety of all houses of worship, and the freedom to practice your religion freely without any fear," the department said in a statement.

In addition to the suspected gunman, who has been charged with murder, at least two others were taken into custody by New Zealand police. Authorities have not released the identities of anyone in custody but did say none of them was on a terrorist watchlist.

In the statement posted online, the suspected gunman cited an array of mass shooters from around the world, including Dylann Roof, the white supremacist who killed nine African-Americans at a Charleston, South Carolina, church in 2015.

Such posts online and the surge of social media are a major factor in radicalizing white supremacists, according to John Cohen, a former senior official at the Department of Homeland Security who ran the department’s efforts to stop targeted violence.

Online platforms allow "like-minded people who in the past were separated by geography" to share ideas and "spread hateful and violent rhetoric," said Cohen, who's now an ABC News contributor.

The statement apparently linked to the New Zealand shooting also mentioned Anders Breivik, the Norwegian extremist who killed 69 people at a children's summer camp in Norway in 2011. As part of the attack, he left behind a statement detailing his far-right views and killed eight others with car bomb in Norway's capital, Oslo.

Breivik's name has surfaced in several cases inside the United States, most recently three weeks ago when FBI agents arrested a U.S. Coast Guard lieutenant described by the FBI as a "domestic terrorist."

According to court documents filed in the case, 49-year-old Christopher Paul Hasson collected a cache of weapons, wrote of his hatred for "muslim scum," and was looking to launch an attack that would bring back "a white Homeland."

Hasson "routinely perused portions of the Breivik manifesto that instruct a prospective assailant to amass appropriate firearms, food, disguises, and survival supplies," prosecutors said in court documents. "[C]onsistent with the directions in the Breivik manifesto, the defendant began the process of targeting specific victims, including current and former elected officials," prosecutors said.

When agents raided Hasson's home, they found 15 firearms and more than 1,000 rounds of ammunition, according to the court documents.

Hasson, however, has only been charged with firearms-related offenses, not any terrorism-related offenses. On Monday, he appeared in a federal court in Maryland and pleaded not guilty to the charges against him.

In New Zealand on Friday, when authorities searched the suspected gunman's car, they found two homemade bombs and several weapons, police said.

ABC News’ Josh Margolin contributed to this report.