The entire U.S. Senate sent a letter on Tuesday urging President Donald Trump’s administration to take “swift action” addressing bomb threats to Jewish community centers and organizations.

In the letter, which was spearheaded by Sens. Marco Rubio (R-FL), Bill Nelson (D-FL), Gary Peters (D-MI) and Rob Portman (R-OH), all 100 senators cited “the deeply troubling series of anonymous bomb threats” made against Jewish institutions.

“We write to underscore the need for swift action,” the letter reads. “We thank you for your recent efforts and ask that you inform us of the actions that your Departments plan to take to address threats against these and other religious institutions.”

Jewish community centers in at least five states and multiple offices of the Anti-Defamation League reported bomb threats on Tuesday in what was at least the fourth wave of threats made against Jewish organizations since January, echoing similar waves of threats on Jan. 9, Jan. 18 and Feb. 27.

White House press secretary Sean Spicer on Tuesday denounced those threats “in the strongest terms.”

“It’s incredibly saddening that I have to continue to share these disturbing reports with you, and I share the President’s thoughts that he fervently hopes that we don’t continue to have to share these reports with you,” Spicer said during his daily press briefing. “But as long as they do continue we will continue to condemn them and look at ways in which we can stop them.”

In their letter, the senators said the threats “are not isolated incidents” and condemned acts of vandalism at Jewish cemeteries in Pennsylvania and Missouri as “completely unacceptable and un-American.”

“We also recognize the anti-Semitic sentiment behind this spate of threats,” they wrote. “These cowardly acts aim to create an atmosphere of fear.”

“We are concerned that the number of incidents is accelerating and failure to address and deter these threats will place innocent people at risk,” the letter concludes. “Your Departments can provide crucial assistance.”