British police Monday identified two of the four men who allegedly tried to bomb London's transportation system last week and issued a fresh appeal for the public's help in tracking them down.

In a televised news briefing, Peter Clarke, head of Scotland Yard's antiterrorist branch, said the four bombs that failed to fully detonated on three subway cars and a bus on July 21 were similar to another bomb that was found abandoned in west London on Saturday.

Shortly after the briefing, police announced the arrest of two more people under Britain's antiterrorism laws, bringing to five the number of suspects detained so far in the bombing investigation. Three persons were arrested Friday and Saturday, but they did not include any of the men suspected of trying to carry out the bombings, officials said. Monday's arrests in north London coincided with police raids on several homes there.

Clarke said the fifth bomb was found by a member of the public in an open area in Little Wormwood Scrubs in west London and that it bore "clear similarities" to the bombs recovered from the failed July 21 attack. He said all five bombs were packed into the same kind of plastic food-storage container and placed in dark-colored backpacks or sports bags. Clarke said the containers that held the explosives -- 6.25-liter models made of clear plastic with white lids and bearing the brand name Delta -- were imported from India and sold at 100 outlets. He urged any shopkeepers who sold five or more at the same time or to the same customer to contact police.

After bombs carried by the four men failed to detonate last Thursday, each of the men managed to escape. The attempted bombings were apparently meant to reprise devastating attacks two weeks earlier in which four suicide bombers set off powerful explosives in backpacks on three subway trains and a red double-decker bus, killing 52 other people and injuring more than 700.

Police subsequently identified the men as three British citizens of Pakistani origin and an immigrant from Jamaica.

Clarke Monday released identities for two of last week's would-be bombers, three of whom he said entered the Stockwell subway station in south London together just before 12:25 p.m. last Thursday.

He said a man believed to be Muktar Said Ibrahim, 27, tried to detonate a backpack bomb on a No. 26 double-decker bus at around 1 p.m., then fled when the bomb failed to explode.

Clarke said police were searching a number of addresses in London, including a house in north London that Ibrahim was "associated with" and recently visited. Ibrahim, who was shown on a surveillance camera wearing a gray T-shirt with a palm tree imprint, a dark jacket and white baseball cap, is also known as Muktar Mohammed Said, Clarke said.

Another suspect was identified as Yasin Hassan Omar, 24, a dark-skinned man who allegedly tried to set off a bomb he carried in a purple backpack on a Victoria Line subway train. Clarke said Omar escaped from the Warren Street subway station after the attempt, vaulting over a ticket barrier.

Clarke did not provide any other information on the two named suspects, and he did not take questions from reporters after the briefing.

A still-unidentified suspect was chased from the Oval subway station by fellow passengers after a bomb he carried failed to detonate, and he ran toward the Brixton neighborhood of south London, throwing away a dark sweatshirt with a "New York" logo, Clarke said.

The fourth suspect, also unidentified so far, escaped from a subway train he tried to bomb near the Shepherd's Bush station, Clarke said. He said the man, who was shown on surveillance video wearing a dark blue baseball cap, probably climbed through a window and ran down the track before making his way through back yards. He was last seen running under the A40 highway, Clarke said.

The antiterrorism chief asked people to come forward if they have seen the men, know their current whereabouts or have any information about them. "The public should not approach them," he said.

Branigin reported from Washington.