Head of London metropolitan police said that four bombs used in three metro stations on July, 21 were similiar

The four bombs that failed to go off July 21 in three London Underground stations and a bus were all packed in plastic food containers - a common construction that could have been used for concealment and convenience, a terrorism analyst said Monday.

According to London police, the four bombs were each packaged inside 6.25-liter (6.60-quart) clear plastic containers sealed with white lids, AP reports.

Initial forensic examination of the four partially detonated devices revealed "clear similarities" with a fifth bomb, discovered July 23 in a west London park and destroyed by authorities, said Peter Clarke, head of the Metropolitan Police anti-terrorist squad.

The containers - labeled "Delta 6250 with Lid" - were made in India and sold at 100 outlets in Britain.

They are not indicative of any particular terrorist organization's techniques, said Brian Jackson, a specialist in terrorist technology at Washington's Rand Corp., a think tank that conducts security research.

"There's certainly a lot of flexibility in the way people can make bombs, so most likely this was just something that was a convenient size for the type of bomb," he said.

The containers would have been used to pack together explosives and possibly shrapnel like nails, and also to conceal the smell of the potent mixture from sniffer dogs, which were widely deployed in London's Underground following the deadly attacks July 7.

"Anything that you can use to keep it in like a Tupperware container, layers of cellophane, could be things that could reduce the release of these vapors," Jackson said in a telephone interview.

Jackson said the containers were certainly large enough to have contained a deadly bomb.

"An explosion in and of itself can do damage, whether or not there's shrapnel," he said.

The July 7 attacks - also on three Tube trains and a bus - killed 56 people, including the four suicide bombers.

Police have not yet said how those bombs were constructed, but would be looking carefully for clues as to whether they were made by the same person, Jackson said.

"Individual bomb makers can develop preferences in the kinds of bombs they make," Jackson said. "For example the IRA's various bomb makers developed signatures for wood boxes or plastic boxes or the wiring."