By Zaffar Abbas
BBC correspondent in Islamabad
The car carrying Pakistani President Pervez Musharraf may have escaped a roadside bomb blast thanks to a radio jamming device, officials say.
An officer stands guard by the bomb-damaged bridge
Five bombs exploded under a bridge in the city of Rawalpindi on Sunday, mere seconds after President Musharraf had passed over it.
The jamming device may have blocked the signal to the remote-controlled bomb.
The president also survived an attempt on his life last year after pledging a crackdown on Islamic militancy.
His personal security has remained under constant review since he joined the US-led campaign against al-Qaeda more than two years ago and declared a war on Islamic extremism in the country.
President Musharraf has himself said that Islamic extremists want to eliminate him.
His security was stepped up in a big way following last year's failed attempt on his life in Karachi, for which three members of a banned Islamic militant group have been convicted.
Musharraf: "It was certainly a terrorist act"
Most of the time two or more identical black Mercedes are part of his motorcade, and it is nearly impossible to make out in which car he is travelling.
There has always been a ban on carrying mobile telephones at his functions.
But lately, security officials have been using a radio-jamming device to block all kinds of wireless communications within a radius of about a couple of hundred metres.
Reports say a vehicle with the jamming device is also part of President Musharraf's motorcade.
There is a strong possibility that it may have blocked the use of a remote-controlled device to detonate the explosives while his car was on the bridge on Sunday.
However, officials are reluctant to comment publicly on the matter.
Meanwhile, nearly 20 people have so far been picked up for questioning from various places in Rawalpindi in connection with Sunday's blast.
Those taken in by the security officials include a cleric from a mosque close to the site of the explosion, as well as some Afghan refugees.
Officials are not at the moment definitely linking these arrests to the assassination attempt.
But they say the placement of the explosives under the bridge and the method used to detonate them suggests that some highly trained people, possibly linked to al-Qaeda, may have been involved.
But so far there has been no real breakthrough in the investigations.