The North Carolina teenager charged with plotting a "mass attack" to kill as many as 1,000 people in the US to support Isis was turned in by his worried father, according to law enforcement authorities.

The criminal complaint charges that Justin Sullivan, 19, initially planned to buy a semi-automatic AR-15 rifle at a North Carolina gun show, and use it to kill people "at a bar or concert." Weapon purchases at gun shows are far less restricted legally than purchases from retailers. But he also spoke of grandiose plans to kill hundreds of people with a biological weapon or bombs, according to the complaint, reports ABC News.

An undercover FBI officer finally made contact with Sullivan in June after his father alerted authorities in April with a 911 phone call. He was worried about his son's behaviour, saying: "I don't know if it is Isis or what, but he is destroying Buddhas, and figurines and stuff."

Sullivan told the agent who reached out to him that he had converted to Islam, and was a "mujahid," authorities said.

Choice of weapons and ammunition

He told the officer he should get "an AR-15 with split-core ammo ... it's fragmenting hollow points ... deadly," authorities said. He also wanted the undercover agent to "just kill a few people so that I know you are truthful," according to authorities. Federal investigators said that among the people he wanted killed were his own parents.

But Sullivan also had much bigger plans, writing the agent that in other operations he imagined "500 is an incorrect body count ... 1,000 is correct," according to the charging document. "I'm thinking about using biological weapons ... coat our bullets with cyanide ... and then set off a gas bomb to finish the rest," he wrote the agent, according to the court papers.

In one of his last conversations with the agent, authorities said Sullivan talked about a second attack, saying: "We could use a U-Haul packed with bombs and then detonate it."

After Sullivan was arrested at his home, he allegedly told interrogating FBI agents that he searched online for nearby places to attack. Authorities believe Sullivan was radicalised by watching Isis videos on social media.

Sullivan is charged with one count of attempting to provide material support to Isis, one count of transporting and receiving a silencer in interstate commerce with intent to commit a felony, and one count of receipt and possession of an unregistered silencer, unidentified by a serial number. The charge of conspiracy to provide material support to a designated foreign organisation carries a maximum potential penalty of 20 years in prison and a $250,000 fine.