Initial reports stated that the package found in the East Midlands, a toner cartridge, had tested negative for explosives. Apparently that was the wrong conclusion for bomb disposal experts in the UK to arrive at however, because Obama later declared that the printer cartridge(s) constituted a "credible terrorist threat to our country" and that "initial examination of those packages has determined that they do apparently contain explosive material."
As with most other such terror alerts, evidence is simply made up as they go along in the interest of political gain:
Barack Obama accused of exaggerating terror threat for political gain.
The main goal appears to have been achieved though because, "the Department of Homeland Security (DHS) said Americans could expect yet more security in and around air travel". That's the point, the reason, the motivation. Everything else is charade.
The fact that one of the packages was found in Dubai is not surprising, given that Dubai is essentially the playground of the Mossad and the CIA and has been ever since the CIA checked up on their spokesman Osama bin Laden in a Dubai hospital just before he didn't carry out the 9/11 attacks in 2001.
But the true litmus test of just how obviously false these terror alerts are is when you hear dyed-in-the-wool mainstream media pundits questioning the timing and likely motivation: