A spokesman said he may have suffered a mere fainting spell, not a heart attack or stroke, and the 67-year-old intended to return straight to work.

Mr Mukasey left the hospital at midday, telling reporters he felt 'terrific', after he fell ill while giving remarks late on Thursday to an annual dinner of the Federalist Society, a conservative group.

The government's chief law enforcement officer was kept for observation overnight at a nearby hospital, and underwent further tests on Friday morning before his release.

Mr Mukasey issued a statement to Department of Justice employees saying he felt fine and would soon be back at his desk.

'As you may have heard, I collapsed briefly last night at the conclusion of a speech. All tests at the hospital have come back with good results, and I feel fine. Accordingly, I plan to report to the Department this afternoon,' he said.

Prior to his release, Justice Department spokesman Gina Talamona told reporters there was 'no indication that he suffered a stroke or any heart-related incident. It really appears to be a fainting spell.

'The attorney general had an uneventful night. He looked good. He's very alert. The doctors are describing him as very fit. All the tests have been reassuring,' she said.

The Justice Department had not reported any recent medical problems for the former judge and prosecutor.

After he collapsed, emergency first aid was administered and he was whisked to George Washington University hospital, where doctors described him as 'conscious, conversant and alert', said Mr Peter Carr, acting director of public affairs.

Video of the incident aired on CNN showed a tuxedo-clad Mukasey slurring his speech and then going rigid, prompting people to rush to his aid to prevent him from falling over.

Mr Mukasey was sworn in as attorney general in November 2007, after President George W. Bush appointed him to replace the scandal-plagued Alberto Gonzales.

Prior to his appointment, Mr Mukasey carved out a no-nonsense reputation during his 18 years as a federal judge in New York, and oversaw several high-profile cases including the 1995 trial of Omar Abdel-Rahman.

Known as the 'Blind Sheikh', Abdel-Rahman was sentenced to life in jail for plotting to blow up New York landmarks, including the United Nations.

As attorney general he has been a vigorous supporter of the Bush administration's use of broad powers to combat terrorism.

Prior to his swearing in, the former judge denounced a 2002 Justice Department memorandum that offered an extremely restrictive definition of torture and authorised, in the name of presidential war powers, some controversial interrogation techniques.

Mr Mukasey, Mr Bush's third attorney general in nearly eight years, had come under fire during the nomination process for refusing to state whether he believed waterboarding was a form of torture. -- AFP