LONDON — A mail bomb intercepted last month at an English airport could have exploded over the East Coast of the United States, British police said Wednesday.

Forensic evidence showed the device, originally sent from Yemen by way of Cologne, Germany, was timed to be detonated about six to seven hours after the cargo aircraft carrying it left the U.K. for the U.S. The package was removed by police in Britain during transit.

"If the device had not been removed from the aircraft, the activation could have occurred over the eastern seaboard of the U.S.," police said in a statement.

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In Washington, the White House said the British finding showed how serious the attack was. Earlier this month, a senior U.S. official had said that while the exact aim of the attack was unclear, evidence pointed to a plot to blow up cargo planes inside the U.S., either on runways or over American cities.

The UPS cargo plane intercepted in England left the country without the package at 3:20 a.m. GMT on Oct. 29 (11:20 p.m. EDT on Oct. 28), two hours after landing, police said. The device was timed to be activated at 9:30 a.m. GMT. (5:30 a.m. EDT), said British police.

Authorities on both sides of the Atlantic said they only narrowly thwarted the plot, in which terrorists in Yemen hid two powerful bombs inside printers and shipped them to addresses in Chicago aboard two cargo planes. The printer cartridges were filled with PETN, an industrial explosive that, when X-rayed, would resemble the cartridges' ink powder.

One bomb was intercepted at central England's East Midlands Airport and the other was discovered at a FedEx cargo facility in Dubai.

The Yemen-based al-Qaida in the Arabian Peninsula has claimed responsibility for the bombs and has vowed to send more explosives-packed parcels.

In Washington on Wednesday, White House spokesman Nicholas Shapiro said: "We greatly appreciate the highly professional nature of the U.K. investigation and the spirit of partnership with which U.K. authorities have pursued this matter."

He praised the efforts of intelligence and law enforcement professionals in the U.K., the UAE, Saudi Arabia and the United States, and said they will continue to work together "to address and counter the threat posed by al-Qaida in the Arabian Peninsula."