US Edition "The target may have been an aircraft and had it detonated the aircraft could have been brought down," she said.
Speaking after a meeting of Cobra, the Government's emergency planning committee, the Home Secretary added: "We do not believe that the perpetrators of the attack would have known the location of the device when it was planned to explode."
She announced an immediate halt to all unaccompanied air freight from Yemen.
"Our investigation remains sensitive," she said. "We are working closely with international partners to increase our understanding of this case and of course to bring those responsible to justice.
"At this stage there is no information to indicate another attack is imminent."
Yemeni authorities said they had seized 26 other suspect parcels at Sana'a international airport and were examining them.
US authorities said the bomb plot bore "all the hallmarks of al-Qaeda".
An international terror alert was sparked after security staff found printer cartridges with wires attached in cargo hubs at East Midlands Airport in the UK and Dubai on Friday.
The two suspected Al-Qaeda parcels from Yemen were intercepted before arriving at what was originally believed to be their destination - synagogues in Chicago.
President Barack Obama said the two parcels were a "credible terrorist threat" and made it clear he suspected Al-Qaeda's Yemeni-based affiliate of being behind the plot.
The White House said it had been tipped off by Saudi Arabia to the threat and was "grateful...for their assistance in developing information that helped underscore the imminence of the threat emanating from Yemen."
Julian Bray, an aviation expert, said: "In the past, terrorists have managed to smuggle packages into cargo operations. This has been made easier, as you don't have the same number of people around as you do in passenger operations.
"However, security still tends to be high in cargo areas in the UK and they are well policed, which may explain why this package was discovered at East Midlands Airport."
Dubai police said: "The investigation into the suspicious packages that came from Yemen through the US delivery company FedEx has shown that (one contained) a computer printer whose ink contained explosive material."
The device contained a highly explosive combination of PETN and lead azide and "was prepared in a professional manner and equipped with an electrical circuit linked to a mobile telephone (SIM) card concealed in the printer."
PETN (Pentaerythritol tetranitrate) is the same substance used by Farouk Abdulmutallab, the would-be 2009 Christmas Day bomber and Richard Reid, the 2001 attempted shoe-bomber.
A Yemeni official said his government had launched a full investigation and was working closely with international partners, including the United States.