While Mr Cook was in Belgrade preaching the need for dialogue and respect of human rights to both Albanian leaders and the Yugoslav President, Slobodan Milosevic, special police units were ripping, apparently indiscriminately, through Albanian villages in Kosovo's Drenica region under the pretext of carrying out an anti-terrorist dragnet operation.
Details of the violence were sketchy and casualties were impossible to estimate accurately because the area was completely sealed off by security forces, but by late afternoon the wounded and dead had begun arriving at hospitals in the provincial capital, Pristina. Sources at the moderate Albanian-language newspaper said that information they were receiving from the area suggested the death toll could run to several dozen people.
Albanians and Serbs have begun to leave Kosovo in recent days for fear of further bloodshed. Albanian reports said that women and children from the Drenica area were trying to escape the conflict on tractors, but that Serb police on the roads were forcing them to go back.
This renewed outburst of police repression was little short of a humiliation for Mr Cook, who had hoped to talk Mr Milosevic into starting a dialogue about greater autonomy for Kosovo with the province's moderate Albanian leadership. Instead Mr Cook, who was acting on behalf of the European Union, found himself powerless to threaten Mr Milosevic with anything worse than a continuation of Yugoslavia's diplomatic isolation. His insistence on implementing a Serb- Albanian education accord reached last year sounded feeble and even trivial in the light of the latest violence.
"I would like to say that I am leaving more hopeful than I was when I arrived," Mr Cook said grimly before boarding his plane back to London. "But I leave with very grave concerns."
He made constant reference to the need for a political solution, not just a police operation to weed out armed Albanian extremists. But he chose not to refer to the fact that many of the victims of a police blitz in Drenica last weekend, in which more than 20 people were killed, had been tortured and gunned down in cold blood without any evidence being produced of their involvement in subversive activities.
Instead, he repeatedly said that Europe could not condone terrorist activities - a perfectly respectable line taken in context, but one that was snapped up by the state media in Belgrade as a partial vindication of the police operations.
The Serbian authorities insisted that their actions were targeted specifically at suspected strongholds of the so-called Kosovo Liberation Army - the shady armed group that has taken responsibility for a string of killings of Serb policemen and Albanian "collaborators" over the past year.
One village that was shelled and set on fire, Prekaze, has often been mentioned as a centre of anti-Serb activism. Another village under fire, Lause, was the scene last November of an ambush on a police convoy that marked the first public declaration of the Kosovo Liberation Army.
Both yesterday and over the weekend, though, the police appeared to be motivated primarily by revenge. Yesterday morning, shortly before the dragnet operation began, a police station in the Drenica area was sprayed with bullets by unknown assailants.
Worryingly, there appears to be an attempt on both sides to radicalise the conflict. Kosovo's traditional Albanian leaders have always preached non- violence, but are now losing ground with an ever-more impatient population clamouring for independence at any price. Mr Milosevic, meanwhile, knows that he can galvanise political opinion behind him on Kosovo and so strengthen his otherwise rather fragile coalition government. The fact that this will entrench his pariah status in the international community does not seem to be of immediate concern.
Mr Cook made no threat to reimpose full sanctions on Yugoslavia and he said that outside military intervention was out of the question - on both points differing from the US position. The one concrete proposal Mr Cook made was to open a European Union office in Pristina to monitor the situation and act as a mediator between the two sides. But it does not appear that he was given any encouragement on this or any other point by his Serbian interlocutors.