The motive for the attacks, which prompted heightened security at media offices and the busy Champs-Elysees shopping avenue, is unclear.
Paris Prosecutor Francois Molins said authorities believe a lone gunman was involved in the shooting at the prominent daily newspaper Liberation, a similar incident at news network BFM-TV, a shooting at French bank Societe Generale, and a brief hostage-taking.
Authorities released video footage of the suspect. Molins said he was wielding a pump-action rifle and wearing a black vest, green shoes with white soles, and a cap. Molins said the suspect's image will be distributed publicly to help with the manhunt.
Monday began with a gunman entering the lobby of Liberation around 10 a.m. and opening fire. A 27-year-old photographer's assistant was in intensive care after being shot near the heart and in the arm, Molins told a news conference.
The shooting prompted cries of concern about attacks on the media. The culture minister called Liberation -- an outspoken left-leaning paper founded by Jean-Paul Sartre that has seen financial difficulties and layoffs in recent years -- a "pillar of our democracy."
Less than two hours after the shooting at Liberation, three shots were fired in front of the headquarters of the bank Societe Generale in the Paris suburb of La Defense, according to Paris police. Societe Generale, based about 10 kilometers (6 miles) northwest of the Liberation offices, said in a statement that a lone gunman opened fire in front of the building, and no one was hurt.
Less than an hour later, a man called police to say he had been taken hostage by a gunman in the town of Puteaux, next door to La Defense. Police said the gunman forced his hostage to drive six kilometers (3Â½ miles) back toward central Paris, then let him go on the Champs-Elysees, a chic and busy shopping thoroughfare.
Police were searching the neighborhood and other sites around the French capital, the Paris police headquarters said.
All this came three days after another incident at BFM-TV, in which an armed man threatened journalists with a rifle in the news network's lobby before fleeing. He left behind unused cartridges, and told witnesses, "Next time, I won't miss," Molins said. It was unclear whether the gunman's rifle malfunctioned or whether he chose not to shoot.
"Taking into account the similarities of these four affairs, in the modus operandi, the clothing of the perpetrator and in the munitions that were recovered, we believe a single gunman is the most likely possibility," Molins said.
The government positioned police at all major media organizations in Paris, according to Interior Minister Manuel Valls. A helicopter flew over the neighborhood that includes the French president's office and the nearby Champs-Elysees avenue.
French President Francois Hollande said he ordered authorities to "mobilize all means to clarify the circumstances of these acts and arrest the perpetrator or perpetrators."
Media watchdog Reporters Without Borders denounced the attacks on the journalism facilities.
Associated Press writers Sarah DiLorenzo and Milos Krivokapic contributed to this report.
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