A German tabloid that reprinted cartoons from the French satirical paper Charlie Hebdo lampooning the Prophet Mohammed was firebombed yesterday as fears grew that the bloodshed in France could signal the start of a wave of attacks in Europe. With security services on high alert after a killing spree in Paris by Islamic extremists, police in the northern German port city of Hamburg said no one was injured in the blaze at the headquarters of the regional daily Hamburger Morgenpost , which caused only slight damage. "Rocks and then a burning object were thrown through the window," a police spokesman said. "Two rooms on lower floors were damaged, but the fire was put out quickly." The Hamburger Morgenpost had splashed the Charlie Hebdo cartoons on its front page after the massacre had occurred at the Paris publication, running the headline "This much freedom must be possible!" Police said two young men seen acting suspiciously near the scene were detained. State security had opened an investigation, a spokesman added. Whether there was a connection between the Charlie Hebdo cartoons and the attack was the "key question", the spokesman said, adding it was "too soon" to know for certain. No one at the Hamburger Morgenpost , which has a circulation of about 91,000, could immediately be reached for comment. "Thick smoke is still hanging in the air, the police are looking for clues," the newspaper said in its online edition. Hamburg is Germany's second city, with a population of about 2.4 million. Media reports said the newspaper's publishers had ordered private security protection for the building in the western district of Othmarschen after publishing the Charlie Hebdo cartoons. German Chancellor Angela Merkel, who was in Paris yesterday to attend a massive march in solidarity with the victims, stressed the need for the exchange of security intelligence among Europe's secret services, particularly among members of the Schengen passport-free zone. Hamburg's Islamist scene came to global attention in 2001 when it became known that three of the suicide hijackers from the September 11 attacks in the United States, including ringleader Mohammed Atta, had lived and studied in the city. Germany's Bild am Sonntag newspaper reported earlier that the bloodshed in France could signal the start of a wave of attacks in Europe, citing communications by Islamic State leaders intercepted by US intelligence. Shortly after the bloodbath in Paris, the US National Security Agency had intercepted communications in which leaders of the jihadist group announced the next wave of attacks, the tabloid said, citing unnamed sources in the US intelligence services. Paris was cited as being the signal for a series of attacks on other European cities, including Rome, the newspaper said, adding however that a concrete plan of attack was not known. The US intelligence services also had information that Cherif and Said Kouachi, the brothers who carried out the massacre at Charlie Hebdo , had contacts in the Netherlands, Bild said. The France attacks could also embolden Germany's new anti-Islamic movement, which plans to rally again today, when analysts expect its ranks to swell by thousands. Leaders of the so-called "Patriotic Europeans Against the Islamisation of the Occident" have asked marchers to wear black armbands and observe a minute's silence for "the victims of terrorism in Paris". Launched in October with a march of just 500 people, the movement has since swelled rapidly, stirring anguished debate in a country whose dark history with Nazism makes any expressions of xenophobia especially worrisome for many.