Nigerian police stepped up security at newspaper buildings on April 27 as authorities probed unprecedented attacks on the media, with two offices belonging to a prominent paper bombed.
The attacks against the influential ThisDay newspaper in the capital Abuja and the northern city of Kaduna killed at least nine people, drew widespread condemnation and came after Islamist group Boko Haram issued threats to the media.
"Yesterday we started sending additional patrols to what we have on the ground with them," Abuja police spokesman Moshood Jimoh said of media buildings.
Precautions were also taken in the economic capital Lagos, which has so far not been targeted by such bombings that have mainly affected the north of Africa's most populous nation and largest oil producer.
"The (commissioner of police) gave a directive yesterday to all area commanders and divisional police officers to beef up security at media houses," said Lagos state police spokesman Joseph Jaiyeoba.
If Boko Haram is confirmed as the culprit, it would be yet another escalation in the group's insurgency that has killed hundreds, with the United Nations, churches and local authorities having already been among its targets.
An online Nigerian news site published a story claiming to have interviewed a spokesman for Boko Haram, saying he took responsibility for the attack while criticising ThisDay and other media outlets for misrepresenting the group.
However, the report on the Premium Times website did not reflect the usual way Boko Haram has communicated with the media and it was not yet clear whether the claim was authentic.
Police in Abuja said they could not comment on who they believed was behind the attacks, although the alleged bomber in Kaduna, who was arrested, was suspected to be a Boko Haram member.
The attack in the capital Abuja saw a suicide bomber drive into the back of the building belonging to ThisDay, one of the country's leading papers.
Five people were killed, including the bomber, a security guard and three passers-by, while eight were wounded at the privately-owned paper, ThisDay said in a statement, which called it an "attack on journalism."
In Kaduna, one of the main cities in the north, a bomb went off outside a complex housing a number of newspaper offices, including ThisDay. An attacker also drove his car onto the premises, which later exploded.
Conflicting details emerged of the Kaduna attack which saw four people killed and at least 19 wounded, a rescue official said.
Secret police said in a statement there were two people in the car in the Kaduna attack, with one caught by civilians as he sought to flee and handed over to authorities. Witnesses reported only one assailant.
The man captured was identified as Umaru Umaru Mustapha from Maiduguri, Boko Haram's base. A police spokesman called him a suspected Boko Haram member.
ThisDay is based in Lagos, but has a major operation in the Abuja.
President Goodluck Jonathan condemned the attacks, as did the United States, saying that they targeted "free speech itself". The New York-based Committee to Protect Journalists and Reporters Without Borders, based in France, also issued condemnations.
"I have always maintained that a terror attack in any part of the country is a terror attack on all of us and indeed the whole world," Jonathan told reporters after being shown the damage by ThisDay publisher Nduka Obaigbena.
The US embassy in Nigeria warned last week that Boko Haram might be planning attacks against hotels or other areas in Abuja, but the government had sought to downplay the concerns.
Nigeria's main journalists' union expressed fears for reporters' safety.
"Yesterday's attacks have confirmed our fear that the media is not safe," Nigerian Union of Journalists President Mohammed Garba said. "Journalists are not safe in Nigeria."
Separately late Thursday, residents reported an explosion at a building at the state university in the northeastern city of Gombe. The building was damaged, but no casualties were reported.
In the nearby town of Bajoga late Wednesday and into Thursday morning, unknown attackers raided two police buildings and a bank, leaving two police officers and a bank security guard dead, authorities said.