KABUL/LONDON (AFP) - Afghan President Hamid Karzai ordered an investigation Thursday into the killing by NATO forces of a former governor in southern Afghanistan, his office said. According to Karzais office, troops broke into the house of Haji Ibrahim, a former district governor in the southern province of Helmand, earlier this week, killing him and arresting six members of his family. Gulab Mangal, the Helmand governor, told Karzai preliminary investigations had revealed Ibrahim was innocent, the presidents office said in a statement. Six members of Ibrahims family who also appeared to be innocent were detained, the statement said. Karzai had ordered Mangal to open an investigation into the killing, the statement added. Daud Ahmadi, a spokesman for the Helmand provincial administration, told AFP an investigation was already under way. The governor has written to the NATO forces inquiring the reason behind the killing. But so far, they have not written back, Ahmadi told AFP. NATOs International Security Assistance force (ISAF) confirmed it carried out a raid but said it had killed an insurgent. As the security force began to search the compound, an insurgent attempted to engage the force with a grenade and was killed, said an ISAF statement Monday, resent to AFP after a query. A British aid worker was killed by a grenade thrown by a US soldier during an attempt to rescue her from Taliban kidnappers in Afghanistan, Foreign Secretary William Hague confirmed Thursday. Hague said that members of the US special forces team involved in the attempt to save Linda Norgrove on October 8 had been disciplined following a joint US-British investigation. Linda Norgrove died as a result of penetration fragmentation injuries to the head and chest, Hague told parliament as he revealed the results of the probe. Hague praised the courage of the US special forces who tried to save the 36-year-old, saying it was an incredibly difficult operation. But he said that early reports that she died because of the detonation of a suicide vest were a matter of concern. Hague said the US soldiers did report their own use of a grenade, but this was not immediately passed up the US military chain of command. Senior officials only learned of it after viewing video footage of the rescue bid. The investigation team found that the failure to disclose information that a grenade was thrown breached US military law, he said. As a result members of the rescue team have been disciplined for failing to provide a complete and full account of their actions in accordance with US military procedure.