Federal investigators looking into the recent wave of bomb threats against Jewish community centers and schools believe the calls to be coordinated and conducted using "spoofing" and voice-masking technology, according to a report from BuzzFeed.

The report cites Paul Goldenberg national director for Secure Community Network, a homeland security initiative formed by a number of Jewish organizations, who claim the investigation is particularly challenging because of the use of technology that has made it difficult to track the origins of the calls.

Often times the bomb threats often appear to come from a familiar number or a known source. This has led investigators to believe the callers are using "spoofing" tactics, which allow a caller to disguise their number. This is a relatively common practice for tricking caller ID, and there are a number of apps and services that simplify the process for the caller.

The callers are also believed to be using voice masking technology to disguise their voices during the calls, making it difficult to identify the caller or confirm if there are multiple people making the calls.

The news comes as the FBI and law enforcement across the country continue to look into the spike in threats against the Jewish community. More than 100 threats in 33 states have been reported since January, and has resulted in students being pulled from schools. Jewish cemeteries have also been subject to vandalism.

On Monday, 30 such threats were reported in at least 18 states, adding to the growing number of attacks against the Jewish community. The calls, which came in clusters during the morning and again during the evening, have thus far not led to any injuries.

CBS News reported earlier this week that law enforcement officials believed the increased number of threats against Jewish community centers and schools to be a coordinated effort. The report speculated that the callers may be using hacked devices to make the calls, using "other people’s phones, communication devices and email addresses to make their threats.

However, as the investigation has continued, the focus has narrowed from a network of callers to perhaps just a lone caller. The New York Times reported the possibility of a lone caller, and noted the calls had been described as having a "high-pitched, rambling voice" that "was disguised and garbled, and warned of a slaughter of Jews."

After being criticized for failing to address the issues facing the Jewish community—and scolding a Jewish reporter for asking about it during a press conference—President Donald Trump condemned the threats during his speech to Congress on Tuesday evening.

"Recent threats targeting Jewish community centers and vandalism of Jewish cemeteries, as well as last week's shooting in Kansas City, remind us that while we may be a nation divided on policies, we are a country that stands united in condemning hate and evil in all of its very ugly forms," the President said.