A major multi-pronged investigation will be under way following the discovery at Heathrow Airport of uranium in a package sent from Pakistan.
Authorities will be pleased their systems have worked and the material was detected but need answers to key questions and quickly, in case more packages are on the way.
There is no innocent explanation for putting uranium in the post, unlike other lower-level radioactive materials used in scientific and other devices.
It is used only for nuclear power and nuclear weapons so; it is fair to assume it was sent for sinister reasons.
Its reported trail of travel is not encouraging either.
"Where it comes from, who it appears to be going to and what it is. All that seems nefarious to me," Hamish de Bretton Gordon, former head of the British Army's chemical weapons unit, told Sky News.
The package originated in Pakistan, according to sources quoted in news reports, a country with nuclear weapons and shadowy links between extremist organisations and elements within its security agencies.
It was destined, we are told, for Iranians in the UK although, so far, no arrests have been made. If those reports are true, they will be deeply worrying to security agencies.
Iranians have been implicated most recently in alleged plots to assassinate targets in the UK, including journalists and dissidents.
The identity of the intended recipients of this package will be crucial to this investigation and establishing motives.
It may sound alarmist, but one of the most likely intended uses for the uranium may well be to build a dirty bomb.
The amounts of uranium involved are too small to make a nuclear device. That would take kilos of the stuff.
A dirty bomb is a conventional explosive device that releases and scatters radioactive material over a wide area.
Packing uranium into a bomb and detonating it in an area packed with people would cause panic and terror. Its main impact would be psychological, sowing worry and alarm over a wide radius.
This is likely to be the most serious radiological incident since the use of polonium to poison Russian, Alexander Litvinenko in 2006.
The assassins' success in smuggling such a dangerous radioactive agent past border control led to an overhaul of controls. This development highlights the success of those changes.
Authorities will be on high alert for the possibility of more such breaches and seeking answers from both Pakistan and Oman to explain their failure in detecting the uranium as it passed through their borders.