The Cabinet Secretary Jeremy Heywood is giving evidence to Public Administration Select Committee about the resignation of former Chief Whip Andrew Mitchell.
Sir Jeremy was asked by the Prime Minister to investigate the claims against Mr Mitchell about the incident involving a police officer at the Downing Street gates when they were first reported last September.
Sir Jeremy has told the committee he did three things.
Firstly, he spoke to Mr Mitchell about the incident in person, secondly, he examined CCTV footage at Number 10 and the Foreign Office.
He also said he sought to meet the person who had sent an email about it to the Conservative MP John Randall - who was Mr Mitchell's deputy at the time - but they did not want to meet him.
Sir Jeremy later reported back to Mr Cameron that it was impossible to ascertain exactly what had happened and the Prime Minister decided to support his embattled Chief Whip.
Britain's most senior police officer has promised a "ruthless" investigation into an alleged conspiracy against Andrew Mitchell.
Andrew Mitchell has written a piece in which he has accused the police of trying to destroy his career in a smear campaign.
Police investigating the row involving Andrew Mitchell have arrested a second man today. See how the "plebgate" events unfolded.