(CNN) -- The suspected killers of Italian and British hostages who had been held in Nigeria have been arrested, an adviser to President Goodluck Jonathan said Thursday in a statement, according to the government-run Nigerian Television Authority.
Reuben Abati, the president's special adviser on media and publicity, said the killers will face the full wrath of the law.
The announcement came shortly after British Prime Minister David Cameron said the Briton and the Italian, who had been held hostage for 10 months in northern Nigeria, died Thursday in a failed rescue attempt. Their captors killed them "before Navy Special Forces could get them," an unidentified official said.
Nigerian security forces, with support from Britain, had launched an operation Thursday to free British citizen Chris McManus and Italian national Franco Lamolinara, Cameron said.
"It is with great regret that I have to say that both Chris and Franco have lost their lives," Cameron said in a statement. "We are still awaiting confirmation of the details, but the early indications are clear that both men were murdered by their captors, before they could be rescued."
Cameron said British and Nigerian authorities had been working to free the hostages since they were kidnapped May 12, 2011.
"The terrorists holding the two hostages made very clear threats to take their lives, including in a video that was posted on the Internet," Cameron said.
The prime minister said authorities couldn't locate the men for months, but eventually "received credible information about their location" and "a window of opportunity arose to secure their release."
"We also had reason to believe that their lives were under imminent and growing danger," Cameron said. "Preparations were made to mount an operation to attempt to rescue Chris and Franco."
Italian Prime Minister Mario Monti said an "unpredicted acceleration of events took place over the last hours." Fearing imminent danger for the hostages, Nigerian authorities activated the rescue. He said Italian authorities were alerted once the operation began.
Both leaders expressed their condolences to the families of the victims. Cameron thanked Nigerian authorities, Jonathan and British authorities for their efforts.
The McManus family said in a statement that Chris "was in an extremely dangerous situation. However we knew that everything that could be done was being done."
The family said they are "devastated" by the development and grieving over their loss.
"Our thoughts are also of course with the loved ones of Chris' colleague, Franco Lamolinara, who are also coming to terms with this truly sad news."
The men worked as engineers for an Italian construction company based in Nigeria, according to Agence France Presse. Gunmen stormed their apartment in Birnin Kebbi, the capital of Kebbi state in northwestern Nigeria, and kidnapped them, AFP said.
Kebbi is on the border with Niger, where al Qaeda in the Islamic Maghreb has claimed abductions of foreign workers.
AFP received a video at its office in the Ivory Coast city of Abidjan last summer in which a speaker said that the two men had been abducted by al Qaeda.
The hostages were "blindfolded and on their knees, with three men holding weapons standing behind them, their faces hidden by turbans," AFP said. The agency reported that video "was accompanied by photographs which showed the men without blindfolds."
It said that the hostages identified themselves and delivered statements "urging their governments to meet the demands of the kidnappers, who they say are from al Qaeda in the Islamic Maghreb."