New evidence has emerged that suggests the 13 members of the Muslim Brotherhood killed by Egyptian security forces in a flat in the Sixth of October area in Cairo on Wednesday were shot to death after being arrested.

Original media reports said that nine men had been killed but pro-Muslim Brotherhood Mekameleen TV said that the number has now increased to 13.

An anonymous security source told the Egyptian daily Watan that Nasser al-Hafi, a former member of parliament, was amongst the dead.

Translation: The 'terrorist' member of parliament Nasser al-Hafi. The coup authorities executed nine in a flat in Sixth of October #Leave_youdog_enough_blood

Abdel-Fattah Mohamed Ibrahim, an MB leader in the Giza governate, was also killed.

Another security official called the Muslim Brotherhood members “armed militants” and said that the group were hiding in a den in the flat. The official maintained that the group opened fire first and that the 13 men were killed in the resulting gun battle.

However, Muslim Brotherhood sources said that the men were well known lawyers and belonged to a legal team that represented imprisoned MB supporters, as well as a committee that supported the families of those killed or detained.

"The relatives of those killed today confirmed they were arrested in the morning before they were killed in cold blood later on with no resistance from their side," according to a member of the Brotherhood. "The photos of the slain members of the Muslim Brotherhood killed today show ink marks of fingerprints at their finger tips, which means their fingerprints had been taken from them as part of their official arrest before they were killed; also there were no signs of bullets on the room's wall or furniture, only bullets in the bodies of those killed."

To support their charges, the Muslim Brotherhood released a video and photos of some of the slain men, which are being widely shared on social media, that they say shows ink on on their hands.

A photo (above) of Jamal Khalifah

Abdulfattah Mohammad (Twitter photo)

According to Egyptian news website Ahram Gate, Hafi, who was a member of the Bar Council, was sentenced to death in absentia and had been in hiding.

The bodies of the victims have been taken to the morgue.

Mohamed Montaser, the spokesperson for the Muslim Brotherhood, said that the lawyers were “executed”.

“The leaders that were executed in the flat were in a meeting [discussing] supporting the orphans of the martyrs,” he said. “They were unarmed, and talk about them clashing with the security is a lie.”

Wael Abbas, a prominent and award-winning blogger and journalist, said that the dead Muslim Brotherhood members were in a meeting to sponsor the families of detainees and those killed.

“What happened is unacceptable and state criminality and murder outside the law,” he tweeted .

'Eliminate his authority'

A statement released in English by the Muslim Brotherhood decried the assassination of the lawyers, who were members of a legal, humanitarian and psyschological support committee, and held Egyptian President Abdel-Fattah al-Sisi fully responsible.

"The Muslim Brotherhood assert that the assassination of its leaders is a turning point that will have its own repercussions by way of which criminal Abd Al-Fattah Al-Sisi is initiating a new phase during which it will not be possible to control the anger of the oppressed sectors who will not accept to be killed in their own houses and in the middle of their families," the statement read.

"The Brotherhood would like to affirm that our martyrs, who were killed by criminal bandits affiliated with the traitor coup perpetrator, were initially detained and rounded up inside the house and then were murdered in cold blood without any investigations or indictments. In this way Egypt has been turned into a state of outlawed bandits."

"While asserting its rejection of the killing and violence in Sinai and in other places, the Muslim Brotherhood is holding criminal Abd Al-Fattah Al-Sisi and his gangs the responsibility for the repercussions of these crimes."

The statement also called on the Egyptian people to "come out in rebellion and in defence of your country, yourselves, and your children".

"This butcher is now perpetrating the biggest massacre ever against this country," the statement continued, referring to Sisi. "Eliminate his usurping authority, destroy the citadels of his oppression and tyranny and reclaim Egypt once more."

Open season on MB activists. Policy of Kill first ask questions later has begun. — Seifeddine Ferjani (@Ferjani9arwi) July 1, 2015

The incident comes hours after militants pledging allegiance to the Islamic State staged a string of large-scale attacks in the Sinai Peninsula. The clashes are ongoing with at the Egyptian army sending in jets to bomb parts of the Sinai. So far, at least 36 mainly soldiers have been killed although some media has put the figure at above 50.

The Egyptian authorities have repeatedly accused the Muslim Brotherhood of backing the militants, although the Muslim Brotherhood denies this.

In light of the attacks, Egypt’s cabinet has now rushed through a new anti-terror law, which will be able to request quicker trials.