Authorities investigating series of hoax calls and bomb threats targeting synagogues across 10 states

Synagogue shooter hit with death sentence The man who killed 11 congregants at a Pittsburgh synagogue was formally sentenced to death Thursday.

Authorities are investigating a series of hoax phone calls and fake bomb threats targeting synagogues across the U.S.

The threats have been occurring across 10 states to at least 25 synagogues, two Anti-Defamation League offices, including their headquarters in New York City, and three Jewish day schools, according to an ADL spokesperson.

The threats began around July 21 and have lasted for approximately four weeks to date, the ADL says, and many of the phone hoax calls appear to be targeting synagogues while they livestream their services.

"We're aware of a small antisemitic and racist group that has been targeting a wide range of religious, public, and educational institutions," ADL spokesperson Jake Hyman told CNN.

The "ADL continues this evening to work with law enforcement and community partners to mitigate the ongoing disruption to Jewish prayer services, as well as additional targets, by a group of online trolls who swat and call in fake bomb threats," the organization said in a Sunday statement.

"These trolls, who employ highly antisemitic language during their calls, have targeted over two dozen synagogues and two ADL offices," the statement said. "They also appear to have expanded their targets, including several African-American churches and a news organization."

Hyman said the ADL has been working very closely with the FBI and law enforcement to investigate if this is an organized group. "We're on this … we're not going to stop until they're caught," he added.

A spokesperson with the New York City Police Department told CNN, "The NYPD is working very closely alongside the ADL and the FBI to fully investigate these incidents as well as carrying out robust outreach with the civic and faith organizations in New York City for ongoing awareness."

NYPD sources say they believe most of the actors creating and phoning in these threats have some expertise in countermeasures to avoid being detected, including concealing IP addresses and using Voice over Internet Protocol phones to make the calls. In recent days, the targets across the U.S. have expanded to include not just Jewish institutions like synagogues, but also churches and mosques.

CNN has reached out to the FBI for comment.

Hyman said he believes these "heinous acts," are an attempt "to disrupt Jewish daily life (and) prayer."

The "callers use very antisemitic language in their calls, they stress you know, how much they hate Jews," Hyman told CNN.

In some instances, the caller will threaten something such as, "There's a pipe bomb here, left in a certain target, (and) this highly antisemitic rhetoric, even when the targets are not Jewish," Hyman said.

In the Sunday ADL statement, CEO Jonathan Greenblatt detailed how the threats have now expanded to other groups outside the Jewish religion.

Hyman said the ADL has offered, "score and guidance and other effective instructions," to institutions and synagogues who are being targeted.

Some of the states where the threats have occurred are California, Pennsylvania, Texas, New York and New Jersey, according to Hyman.

Three incidents in the Los Angeles area were reported, he said.

Citing a historic rise in antisemitic incidents, the ADL of Los Angeles and the Jewish Federation of Greater Los Angeles announced Wednesday "an integrated, security partnership known as the Southern California Jewish Security Alliance," which aims to "better prevent and respond to antisemitism in the area."

Hyman said the threats, which mostly came in on weekends, have now continued into the week. "We've seen it happen in the last two days," he said. "It's continuing. I want to stress that it's continuing."