The attack on journalists in Paris is not an isolated incident but part of a broader attempt to muzzle the press. At least 158 reporters and photographers have been killed while doing their jobs since 2011, the worst three-year period on record.

"There is a global battle over the freedom of expression," said Joel Simon, executive director of the New York-based Committee to Protect Journalists, which has been tracking press killings since 1992. "Journalists are unquestionably under greater threat than ever."

Newspapers and magazines across Europe and the Middle East are bracing for violence in the aftermath of the shooting at the offices of Charlie Hebdo, a satirical weekly, that left 12 dead. Other newspapers in Paris have stepped up security, as did Jyllands-Posten in Copenhagen, the publisher of controversial cartoons of the prophet Muhammad in 2005. In Madrid, the offices of the El Pais newspaper were evacuated after security guards detected a suspicious package. The Financial Times said it was raising security levels.