Investigators found no physical evidence of loud explosions heard Monday near the Pakistani capital and officials said it may have been a sonic boom caused by fast-moving aircraft.

The blasts were heard in Rawalpindi and the capital, Islamabad, two cities about 7 miles (12 kilometers) apart. Rawalpindi houses the headquarters of
Pakistan's army and the residence of President Pervez Musharraf, a key U.S. ally in the war on terror.

'All sensitive installations are clear,' said Rao Mohammed Iqbal of the Rawalpindi police.

Hours after the explosive sounds echoed through the area, there were no reports of any detonation site having been found.

Air Cmdr. Humayun Waqar Zephyr, a Pakistan air force spokesman, said authorities were looking into whether an aircraft caused a sonic boom after taking off from an air base outside Islamabad.

But 'there was no flying activity' in the immediate vicinity of the two cities, he said.

He said that in clear weather conditions the boom of a jet crossing the sound barrier can reach Islamabad from nearby air bases. He declined to specify any distance for any base near the capital.