The spate of mail bombs sent to prominent Democrats this week has spurred news organizations to step up security measures, in another disturbing sign of the polarization that has created new dangers for journalists covering politics and divisive social issues.

The FBI and other law enforcement agencies have mounted a nationwide investigation into the source of the explosive devices addressed to such figures as former President Barack Obama, former Secretary of State Hillary Clinton and former Vice President Joe Biden. A package addressed to former CIA director John Brennan that was sent to CNN forced the evacuation of the Time Warner Center on Wednesday. Actor Robert De Niro was targeted early Thursday morning in a package addressed to him and sent to the office of his Tribeca Enterprises in New York.

The targeting of CNN and prominent critics of President Donald Trump has fueled speculation that the mail bomb spree is politically motivated.  President Trump’s relentless attacks on mainstream news organizations has spurred fears among journalists and others that activists and extremists will turn to violence. In June, five reporters were gunned down at the offices of the Capital Gazette newspaper in Annapolis, Md.

The emergence this week of what appears to be a coordinated plot to both harm and intimidate prominent Trump critics has forced news organizations to tighten security measures at their facilities.

A rep for the New York Times said it has implemented additional security measures in light of the “heightened degree of threat.” The Los Angeles Times has done the same after enduring a scare over a suspicious package at its new headquarters in El Segundo on Wednesday.

“Given the reports about the package sent to former CIA chief John Brennan at CNN, and those sent to Obama, Hillary Clinton and others, our security and facilities teams are being extra careful with mail and packages, as well as with any visitors to the building,” a Los Angeles Times rep said in a statement. “Yesterday, law enforcement and hazardous materials teams thoroughly screened a few pieces of mail and inspected our mail room facility. Both were cleared and no hazardous materials were found. We generally have security on site at the property and in the building 24 hours a day. We also have metal detectors at the building entrance, where all staff and visitors are required to check in. Mail and deliveries are screened and sorted in a separate facility.”

According to Matt Doherty, senior VP of threat violence and risk management at security firm Hillard Heintze, among the best practices that news organizations should engage in as they look to ramp up security are to have a secure reception area, a visitor log, and open line of communication with local law enforcement.

“If there’s someone who’s a nuisance in the local community, a frequent caller, make sure that your first line of defense is that reception area and security guard,” he says.

For mail screening, which newsrooms are likely to now increase, Doherty adds that the ideal is for mail screening to take place off-sight. “If you don’t have it off-sight, there are technologies available to screen your mail in a secure area within your newsroom building,” he says. “But the best practice is off-site screening.”

For prominent TV news personalities, there is greater consideration of the need for security guards and other muscle to accompany them while on assignment. Those kind of precautions have been increasingly employed for journalists sent to cover war-torn regions well outside U.S. borders. But nowadays, there’s increasing concern about the menace at home. “You have to wonder what your people might be walking in to” when reporting on politics and social concerns on the domestic front, said a veteran TV news executive at a major cable outlet. “

The pipe bomb incidents of the past week have set nerves on edge. The gravity of the situation that CNN staffers faced on Wednesday was not lost in its new parent company, AT&T. The telco giant on Thursday unveiled a $250,000 donation to the Committee to Protect Journalists.

“Journalists around the world do the important work of holding leaders accountable, exposing the truth, and telling the stories that need to be told. Threats to journalists are, at the very core, threats to freedom and liberty,” said AT&T chairman-CEO Randall Stephenson. “In light of numerous recent events around the world where journalists have been threatened, harmed, and even killed, we’re pleased to support the Committee to Protect Journalists, whose work is more important than ever.”

Gene Maddaus and Daniel Holloway contributed to this report.