ABUJA, Nigeria — Muslim extremists may have planted the bomb that exploded at an army barracks in Nigeria's capital, the country's president said Saturday after the latest attack in a nation beset by violence in recent weeks.No one has claimed responsibility for Friday's explosion, which killed at least four people and wounded 21 as they celebrated New Year's Eve in the barrack's open-air beer garden and market. President Goodluck Jonathan promised people attending a church service in Abuja that those responsible would be found.However, his vague remarks appeared to indicate that security agencies remain ill-prepared to halt such violence as the nation nears what could be a tumultuous April election."Some people say they are politicians, some say they are religious fanatics, but to me they are pure criminals, they are (the) ones demons are using these days, not only in Nigeria," Jonathan said. "For those of you who have time to listen to world news on Al-Jazeera or CNN, you will see that terrorism is crisscrossing the whole world."On Saturday, US President Barack Obama condemned the attack, offered his condolences to the victims and their families, and offered to help Jonathan's government bring the perpetrators to justice."Killing innocent civilians who were simply gathering — like so many people around the world — to celebrate the beginning of a New Year further demonstrates the bankrupt vision of those who carry out these attacks," Obama said in a statement.The blast struck the barracks, called the Mogadishu Cantonment, around 7:30 p.m. Friday in an area of market stalls and beer parlors referred to locally as a "mammy market."