The opening day of the long-awaited inquests also heard recordings of a flurry of phone calls to the emergency services, gradually revealing the devastation wreaked by the bombing of three Underground trains and a bus.
The hearing heard that what became the largest terror attack on British soil may have been timed for the morning of July 6. Later that day, London beat Paris in the vote on hosting the 2012 Olympics.
Hugo Keith, counsel for the inquests, said: “It may have been that the attack was originally planned for a different day.”
Plot ringleader Mohammed Sidique Khan visited a hospital with his wife on July 5 due to complications with her pregnancy and she miscarried on the day of the attacks.
His mobile phone, recovered from the train tunnel wreckage, revealed that several calls were made to the other three on July 5 and 6.
He sent a text message at 4:35 am on July 6, apparently abandoning a meeting.
“Having major problem. Cannot make time. Will ring you when I get it sorted. Wait at home,” he wrote.
They might also have been plotting to hit busier stations — including Westminster, next to parliament — but a 24-minute delay on the train to London meant they could only get so far before the detonation time of 8:50am.
The chaos of July 7 was recalled when the inquest was played a series of telephone calls between the Underground network control centre (NCC), the stations affected and the emergency services.
What started as reports of a bang at one station, Aldgate, possibly a power problem, swiftly escalated with casualties emerging from the tunnels and emergency services rushing to the stations.
Aldgate station supervisor Celia Harrison said in one early call: “We’ve just had a big explosion.
“We’ve had thick smoke coming from the tunnel, we’ve had customers on the track.
“Will you please get as many ambulances as you can here, we’ve got injuries.”
Other calls tell of victims emerging covered in blood and soot.
“Something’s gone badly wrong down there,” said the Edgware Road station manager.
Another from the NCC said: “I’m leaning towards terrorist at the moment, but I can’t tell you that.
“We have possible fatalities.”
Proceedings began with a moment of silence for the victims, whose names were read out.
In his opening statement, Keith, whose role is to present information to the coroner in the hearings, said the bombs “detonated amongst the innocent and the unknowing, indiscriminately killing and maiming passengers who were simply going about their daily business.
“The slaughter caused by the bombs caused not only death, devastation and mutilation but unleashed an unimaginable tidal wave of shock, misery and horror on their families and their loved ones.
“They were acts of merciless savagery and one can only imagine at the sheer inhumanity of the perpetrators.”
He warned relatives of the dead that some of their questions may never be answered.
But he added: “We are confident that these inquests can deliver meaningful conclusions about the indirect as well as direct causes of the deaths.”
The inquests, presided over by judge Heather Hallett, acting as coroner, had been delayed pending the trials of the alleged conspirators of the four suicide bombers.
The 484 witnesses will include survivors and members of the emergency services.
The inquests can examine whether police and the domestic intelligence service MI5 could have done more to prevent the attacks.
MI5 has sought to block much of the questioning on the grounds that it would require the disclosure of secret files that would threaten national security.
Survivors and bereaved relatives have said they want to ask why security officials did not act to stop Khan and Shehzad Tanweer despite having monitored them previously.
Speaking before the inquests started, Ros Morley, whose husband Colin, 52, died in one of the Underground bombings, said: “Innocent citizens in the UK and worldwide need to know that they are protected now and in the future.”
Inquests into the bombers’ deaths will be held separately at the request of their victims’ families.
The four near-simultaneous attacks unleashed a wave of unease about the threat of homegrown extremism and how well integrated Britain’s Muslim community is.
The hearings are expected to last until March.