Donald Macintyre's Sketch: The most shocking revelation yet in the Stephen Lawrence saga. Until the next one

Perhaps the aptest – and the most chilling – five words uttered by Theresa May in the Commons came towards the end of her statement when she was saying how “deplorable” it was that Stephen Lawrence’s family had to wait 21 years for the truth to emerge about the wilfully botched police investigation into his death.

"Indeed," she added, "it is still emerging."

Her choice of tense was judicious. Incredibly, thanks to a prolonged Metropolitan Police campaign of cover-up, lies and what Jack Straw called "venality, probably in the upper reaches" of the force, we still don't yet know the full story of how its officers obstructed justice. Or if other cases could yet be unravelled following the malpractices laid bare. 

As often on Thursdays, the Commons back benches were a just a quarter full to hear what Straw called "one of the most shocking and serious statements I have heard by any minister from any party over the whole of the 35 years I have been in this House".

But that didn't prevent a mild frisson in the chamber when Ms May announced, with just a touch of theatre, a public inquiry into the working of the secretive and now disbanded Special Demonstration Squad.

It was the Squad which had furnished, in Mark Ellison's lawyerly prose, "a [Met] spy in the Lawrence family camp during the course of judicial proceedings in which the family was the primary party in opposition to the [Met]".

Complimented by the usually spikily partisan Labour MP Heidi Alexander not only on the content but her "tone", the Home Secretary did well, sombrely reflecting what she herself called the report's "profoundly shocking" findings.

To Ms Alexander's interesting question about whether any of the officers who knew about the allegations of corruption during the initial murder investigation - scandalously withheld from Macpherson - were still employed by the Met, she was wisely cautious, promising to write to the MP when she was "absolutely certain of the facts".

But there was nothing cautious about her emphasis on the need for a "change of culture" in the police, or of her dismay at the impact on the Met's own corruption investigations of the "mass shredding" of "key evidence" in 2003.

Like her opposite number Yvette Cooper, she was careful to pay ritual tribute to the "majority" of officers "who conduct themselves honestly and with integrity". But she did not mince her words in wondering what "the thinking was of somebody who thought it was right not to refer" the corruption allegation against Detective Sergeant John Davidson to Macpherson, adding: "I find it absolutely incredible that further reference did not take place."

Ms May was also forthright in telling MPs "that in policing, as in other areas, the problems of the past have a danger of infecting the present and can lay traps for the future. Policing stands damaged today." And the story isn't over yet.