The rhetoric on social media following the murder of healthcare CEO Brian Thompson in New York earlier this month has been “extraordinarily alarming”, US Homeland Security Secretary Alejandro Mayorkas says.

“It speaks of what is really bubbling here in this country, and unfortunately we see that manifested in violence, the domestic violent extremism that exists,” he told CBS’s Face the Nation on Sunday.

Some on social media have celebrated Luigi Mangione, the man accused of shooting dead Mr Thompson, and shared anger at America’s private health insurers.

Mayorkas said he was “alarmed by the heroism that is being attributed to an alleged murderer of a father of two children on the streets in New York”.

Mr Thompson, the 50-year-old CEO of the largest US health insurer UnitedHealthcare, was gunned down outside a Manhattan hotel early on 4 December triggering a massive manhunt for the killer.

Mr Mangione, 26, was arrested days later in Pennsylvania and flown to New York where he is facing both federal and state charges, including first-degree murder in furtherance of terrorism.

Investigators accuse him of carrying out a targeted killing, pointing to evidence that suggests a long-held animosity towards the US healthcare industry. On social media, support for Mr Mangione has often been accompanied by grievances and complaints with the health insurance sector.

“We have been concerned about the rhetoric on social media for some time,” Mayorkas said on Sunday. “We’ve seen narratives of hate. We’ve seen narratives of anti-government sentiment. We’ve seen personal grievances in the language of violence.”

Mayorkas, whose homeland security department is in part responsible for protecting Americans from domestic terrorism, said his department sees a “wide range of narratives” that “drive some individuals to violence.”

“It’s something that we’re very concerned about,” he said. “That is a heightened threat environment.”

But the 65-year-old, whose time at the helm of the department will end next month, stressed that Mr Thompson’s killing was “the actions of an individual [and] not reflective of the American public”.

Mr Mangione will remain behind bars in New York as his lawyers said last week that they would not present an application for bail. He is in federal custody at the Metropolitan Detention Center Brooklyn, the same facility where Sean ‘Diddy’ Combs is being held.

He will likely be assigned a roommate and have daily visits from medical and psychological services, law enforcement sources told the BBC’s US partner CBS.

While New York does not have the death penalty, he faces four federal charges, including murder and stalking, which could make him eligible for the punishment. He also faces multiple state charges.

He is expected to be arraigned on those state charges in New York on Monday. Mr Mangione faces 11 counts, including murder in the first degree and murder as a crime of terrorism.