According to police, the 38-year-old Congressman for Rhode Island crashed his Ford Mustang into a security barrier close to the Capitol building at 2.45am.
The report suggested Mr Kennedy was drunk, that his driving ability was impaired and that the car had been travelling at high speed. The Congressman's eyes were said to be red and watery, his speech slurred, and his balance unsure.
Yesterday, Mr Kennedy flatly denied he had been drinking, blaming his condition on two prescription drugs he was taking Phenergan, used to treat gastroenteritis, and the sleeping medication Ambien.
And on the steps of Capitol Hill last night he announced that he was immediately to seek treatment for addiction to prescription pain medication at the Mayo Clinic in Rochester, Minnesota.
Mr Kenndey was returning to the Capitol at that late hour, he said, because he thought he was required to take part in a vote. "I never asked for any preferential treatment, I am going to co-operate fully with the police."
But the police union has complained that supervisors called to the scene by the reporting officer had not allowed a more detailed investigation, including alcohol tests. A breath test "certainly would have showed he wasn't on anything, wouldn't it?" Lou Cannon, a police union official, toldThe Washington Post.
By common consent, Mr Kennedy has been an effective Congressman for Rhode Island, who has worked hard on mental health issues. But his past has seen troubled episodes, including a spell in drug rehabilitation in 1986, and he was once involved in a scuffle with a security guard at Los Angeles airport. By his own admission, he has also suffered from clinical depression.
The affair adds yet another footnote to the saga of a family that once was America's closest equivalent to royalty, as famous for its tragedies and troubles as for the political achievements of its members.
In political terms, the Kennedys have long since been eclipsed by the Bushes and the Clintons. The clan's last standard-bearer of real stature is 74-year-old Ted, a liberal lion of US politics who has represented Massachusetts in the Senate for 44 years. But he is equally remembered for his two murdered brothers John, the 35th president, and Robert, New York Senator and presidential candidate who was murdered in 1968.
Of course, Ted is also remembered for the car accident in 1969 that ended his own hopes of the presidency. The similarities with events this week are limited but hard to ignore. On 18 July 1969, after a party at Chappaquiddick in Massachusetts, Ted Kennedy drove his car off a low unlit bridge into water. He escaped but the staffer travelling with him, Mary Jo Kopechne, drowned.
That affair became the butt of cruel humour, and this new accident involving Patrick promises more of the same. "A Kennedy, a car accident, and no alcohol now that has never happened," Jay Leno joked on his NBC Tonight show.