The July 7 attackers used hair bleach and citric acid to prepare the explosives that killed 52 innocent people in London four weeks ago and set off their bombs with mobile phones, according to the New York Police Department.

Precise details about the composition of the bombs and their method of detonation were made public yesterday at a conference in New York for business leaders and transport police at Police Plaza in Lower Manhattan.

Ray Kelly, the New York Police Commissioner, told a mixed audience of police and private security officials from New York's banks, hotels and storage facilities that the explosives used on July 7 were not military grade, as was originally thought, but were made of products that can be bought on any high street.

"Initially it was thought that perhaps the materials were high-end military explosives that were smuggled, but it turns out not to be the case," said Commissioner Kelly. "It’s more like these terrorists went to a hardware store or some beauty supply store."

According to NYPD investigators, some of whom flew to London within hours of the attacks on July 7, the three bombs that exploded within seconds of each other on the Underground system were probably detonated by mobile phones with their alarms set for 8.50am.

In an unusually detailed briefing, officials from the NYPD's large anti-terrorism department, said that the bombs used a peroxide-based explosive called HMDT, or hexamethylene triperoxide diamine. HMDT can be mixed from mundane ingredients such as hydrogen peroxide (hair bleach).

The only unusual piece of equipment the bomb-maker needed to produce large quantities of HMDT was a commercial refridgerator, because the explosive degrades if it is left at room temperature.

Yesterday, NYPD officials said that an expensive fridge was found in the otherwise rundown flat in Dewsbury, on the outskirts of Leeds, where investigators believe the bombs used on July 7 were built, and that the devices were brought to Luton in cooler boxes in the boots of two cars.

"In the flophouse where this was built in Leeds, they had commercial grade refrigerators to keep the materials cool," said Michael Sheehan, the NYPD's Deputy Commissioner of Counter Terrorism.

Mr Sheehan told the conference that the presence of sophisticated cooling equipment in an unlikely place could be "an indicator of a problem".

The NYPD said that experts were still analysing the links between the devices used on July 7 and July 21. Mr Sheehan said that the bombs used in the failed attacks contained similar explosive compounds to those used four weeks ago, but that they had been hand-activated rather than detonated by timers.

Mr Sheehan warned his audience of representatives from large New York institutions and police from Washington, Philadelphia, Baltimore that there were troubling signs that the July 7 bombers had links to "organisations" that he stressed might be operating in America.

"We know those same types of organisations that they’re affiliated with are very much present in New York City," said Mr Sheehan. "That’s something we’re studying very, very carefully ... This could happen here."

After the briefing, Paul Browne, an NYPD spokesman, said the department had clearance from British authorities to present the information about the explosives used on July 7.

New York has been the source for a series of revelations about the London bombings. The NYPD has been working closely with the Metropolitan Police and has increased security on public transport in New York City, with random bag searches instigated at major stations.

Last week, Commissioner Kelly thanked Scotland Yard for being "very open and cooperative with us".

On July 26, the Associated Press reported that an NYPD explosives specialist had recently returned to New York "with a detailed analysis of the bomb-making techniques used in London".

The following day, ABC news, which is based in New York, published several leaked photographs showing bombs left in the car abandoned by the bombers at Luton Station as well as images taken of the inside of the Tube trains destroyed by the attacks.

Scotland Yard was not prepared to discuss the NYPD briefing this morning.

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