An Israel-American teen has been charged with 31 counts after allegedly threatening Jewish centers, schools and commercial flights across the US, hiding his voice with voice-modulating software.

NBC News . Michael Ron David Kadar, 18, was arrested in Israel in March. A dual US citizen, he is being charged in Georgia and Florida, according to

In one February 2017 call, Kadar promised that 'in a short time, a large number of "Jew children" were going to have their heads blown off from the shrapnel,' an FBI report alleged.

Kadar is Jewish, an official told ABC 7.

Arrested: Michael Kadar (pictured in an Israeli court in 2017) was arrested after allegedly threatening US Jewish organizations over the web from Israel. He is reportedly Jewish

The calls date back as far as August 2015, and frequently saw Kadar threatening 'bombs' and 'bloodbaths', the FBI said.

He disguised his identity by using text-to-voice services and voice modulation software.

And he equipped his home with an antenna that allowed him to use multiple wi-fi connections in his neighborhood to further hide his identity.

But the FBI were able to track the calls after serving 100 subopenas and search warrants to various internet providers.

They then tracked the time of day and phrases to pinpoint the suspect.

According to the Florida indictment, which covers calls made between January 4 and March 3, when the voice modulation was stripped out, the recordings retained a speech impediment similar to Kadar's own.

And Israeli police said that his home contained recordings of the calls, organized by date, on a USB drive.

He has been charged with 28 counts of making threatening calls and conveying false information to police in that state.

The Georgia case involves Kadar allegedly reporting a fabricated home invasion and shooting in order to get a location 'swatted' - that is, raided by police as a 'prank'.

There, he has been charged with three counts of making threatening calls, conveying false information and cyberstalking.

He is also accused of making threats against Jewish centers in Australia and New Zealand.

Threats: Kadar allegedly threatened to blow up Jewish children at centers and schools. He used a voice modulator to hide his identity, the FBI claims. He's a dual Israeli-US citizen

Both Kadar's mother and lawyer claim that his actions were likely influenced by his autism and an inoperable brain tumor.

'The best surgeons in the world refused to operate on him due to immediate danger for his life,' his lawyer, Shira Nir, told NBC News.

At the time of Kadar's arrest, his lawyer had claimed that his condition had stopped him going to elementary or high school, and had got him out of compulsory enlistment in the Israeli army.

The Jewish Community Center Association said in a statement that it was 'troubled to learn that the individual ... is reportedly Jewish.'

In a statement, US attorney Jeff Sessions said in a statement: 'Today's charges into these violent threats to Jewish Community Centers and others represent this Department's commitment to fighting all forms of violent crime.

'These threats of violence instilled terror in Jewish and other communities across this country and our investigation into these acts as possible hate crimes continues.'

Johnathan A Greenblatt, CEO of the Anti-Defamation League, warned: 'Anti-Semitism in the US remains a very serious concern.

'No arrests have been made in three cemetery desecrations or a series of other anti-Semitic incidents involving swastika graffiti and hate fliers.

'JCCs and other institutions should not relax security measures or become less vigilant.'