The FBI is investigating claims that the alleged gunman in today's Walmart Texas shooting posted a manifesto citing the Christchurch mosque massacres not long before the attack, according to reports.

Local media say 18 people were killed and at least 24 people injured after the shooting at the border town of El Paso.

The alleged shooter - named as Patrick Crusius, who has just turned 21, of Dallas - is in custody. It is understood no shots were fired by police during his arrest.

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Gunman opens fire in Texas Walmart, killing multiple people

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A manifesto containing white supremacist content was apparently posted online before today's Walmart attack, on the same forum used by the accused Christchurch shooter.

The FBI is now examining the manifesto, Texas media reported.

In it the author claims to support the actions of the person responsible for the shootings at Christchurch's Al Noor Mosque and the Linwood Islamic Centre on March 15, and also supported the sentiments expressed in the Christchurch shooter's manifesto.

The document claimed similar white supremacist reasons for today's shooting. The author said they had spent less than a month preparing and listed the firearms they planned to use.

They also said they expected to die, which would be preferable to being captured by police. Today's alleged gunman was arrested and appeared uninjured.

Police spokesman Robert Gomez says investigators are still trying to determine what happened during today's attack.

He says multiple people were shot, but he didn't say how many.

U.S. Customs and Border Protection officers at the scene of a shooting at a shopping mall in El Paso, Texas. Photo / AP
U.S. Customs and Border Protection officers at the scene of a shooting at a shopping mall in El Paso, Texas. Photo / AP

NBC News reports that a University Medical Center of El Paso official said it had received at least 10 seriously injured victims and that victims had also been taken to other hospitals.

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El Paso, which has about 680,000 residents, is in West Texas and sits across the border from Juarez, Mexico.

A previous shooting at a synagogue in California in April was also inspired by the Christchurch gunman's actions. One woman was killed and three people injured in the attack on the Poway Synagogue on April 27.