Four of the most dangerous prisoners in Iraq escaped overnight on Wednesday from American custody in a heavily fortified prison on the edge of Baghdad, an embarrassing security breach that led to an intensive manhunt, American and Iraqi officials said Thursday. The four prisoners, who were not identified, were among more than 200 that Iraq asked the U.S. military to continue to hold because of their prominence and the potential threats they posed after the Iraqi government assumed control of the last American-built prison here, formerly known as Camp Cropper. The 200 include former members of Saddam Hussein's government and senior foreign and Iraqi insurgents. The newly installed American commander in Iraq, Lt. Gen. Lloyd J. Austin III, met with Prime Minister Nouri al-Maliki on Thursday night to discuss the prison break. 'We stressed the necessity of tightening security and closing any gaps for criminals to escape,' al-Maliki's spokesman, Ali al-Mousawi, said of the meeting in a telephone interview. The prisoners escaped during what appeared to be a larger attempted prison break late Wednesday evening, though officials declined to provide many details, citing the need for security as American and Iraqi forces intensively searched the areas in and around Baghdad International Airport on Thursday. The four were discovered missing after two other prisoners were caught trying to escape, prompting a head count of all the prisoners in the remaining American-controlled section of the prison, the American military said in a statement. It was not clear how the four had managed to get out of a prison located inside a bigger military base, one of the most secure places in all of Iraq. The Americans and the Iraqis began a joint investigation into the escape, according to the military's terse statement. They also joined in searching for the four escapees, focusing on a neighborhood near the airport. By late Thursday, the four remained unaccounted for. 