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Police discovered fragments of a "silent conversation" writing in a document on Dart's laptop and then deleting the text.
Counter-terrorism teams also believe that the pair used the same tactic walking down the street with a mobile phone.
Police discovered fragments of text on Dart's laptop that revealed that the pair had used the computer to have a "silent conversation" to avoid possible surveillance bugs.
They would open a Word document and take it in turns to type, before deleting the text and mistakenly assumed that none of it would be stored on the machine.
However forensic experts were able to plough through 2,000 pages of computer code to decipher fragments of what was said.
These included Mahmood making a reference to Wootton Bassett and then adding "if it comes down to it it's that or even just to deal with a few MI5 MI6 heads".
Mahmood and Dart were both given extended sentences and will serve two-thirds of their prison terms rather than half, and will spend five years on licence.
Dart refused to stand when he was sentenced, saying: "I don't wish to stand up, I believe ruling and judging is only for Allah."
The judge said that they were all "committed fundamentalists" who would have been prepared to kill.
He told Dart and Mahmood:
Three British Islamic extremists including a white Muslim convert and a former police community support officer have been jailed for terrorism.
Richard Dart, Jahangir Alom and Imran Mahmood, were sentenced at the Old Bailey for engaging in conduct in preparation of acts of terrorism.
Dart was jailed for six years, Alom for four years and six months and Mahmood for nine years and nine months.
Mr Justice Simon told the trio they held "radical Islamist beliefs and have shown yourselves to be committed to acts of terrorism".
They admitted the offence between July 2010 and July last year at a previous hearing last month.
White Muslim convert Richard Dart, 30, was jailed for six years at the Old Bailey today for terrorist offences. He refused to stand as he was sentenced.
Today the sentencing process, which could take until tomorrow, began at the Old Bailey.
Prosecutor Jonathan Laidlaw QC told the court:
"The method employed as the police, with the help of computer experts, would subsequently discover, involved Dart and Mahmood sitting together at a computer and opening a Word document on the computer to conduct what in effect was a silent conversation.
"Having had that discussion by typing into the document, the document was then deleted by one or other of the defendants, without having been saved and as far as the defendants were concerned the document would therefore be destroyed forever.
"They plainly were under the misapprehension that the text once deleted could never be recovered."
- Police recovered fragments of text from Richard Dart's laptop which referred to Wootton Bassett
- Dart changed his name to Salahuddin al-Britani and became involved in extremism after moving from his home town Weymouth in Dorset to east London
- His beliefs were brought into the spotlight as part of a television documentary "My Brother the Islamist" made by his step brother Robb Leech
- In the film he was seen preaching in the street, attending extremist lectures
Three men, including a white Muslim convert, are due to be sentenced after pleading guilty to plotting a terror attack in the former military repatriation town of Wootton Bassett.
Richard Dart and his co-conspirators Jahangir Alom and Imran Mahmood admitted to the plot last month. Dart and Alom travelled to Pakistan to try to get terrorist training, and took advice from Mahmood who had already visited the country.
Dart, a former BBC security guard, also discussed bomb making and potential targets with Mahmood, which included the town of Wootton Bassett.