A woman from east London who was in the aid flotilla attacked by Israeli commandos is in a prison in the southern Israeli city of Be'er Sheva.
Alexandra Lort-Phillips, from Hackney, was one of the nine Londoners heading for Gaza in the six-ship flotilla.
Her brother Charlie Lort-Phillips said he was disturbed that he learnt about her incarceration first from Facebook.
He said the family had panicked not knowing what happened to her since the attack in the early hours of Monday.
Mr Lort-Phillips said the last time he heard from his 38-year-old sister was when she updated her blog on the ship.
The flotilla, carrying 10,000 tonnes of aid, left the coast of Cyprus on Sunday and was due to arrive in Gaza on Monday, when it was attacked.
He said: "I received a phone call at about 7.20 from my mother who was quite panicked, who said along the lines of 'its all gone horribly wrong and people are dead'."
He said it was only on Tuesday afternoon he heard about his sister's whereabouts.
"She is in Be'er Sheva prison. I found that out first from a message on Facebook and the fact that we hadn't been called initially is quite disturbing.
"She is totally independent-minded. She was offered a place and she took it as she felt it would be the right thing to do.
"She left her job, she left someone in her flat and she has gone off to see if she can try and play her part in what she sees as a massive injustice," he said.
Mr Lort-Phillips has been informed that the British embassy in Israel has been in contact with his sister and she is safe.
Londoners believed to be among the 40 British nationals taking part in the aid flotilla: Kevin Ovenden, from Newham, east London; Nicola Enchmarch, from Wimbledon, south west London; Nader Daher, from Hendon, north London; Alexandra Lort-Phillips, 38, from Hackney, east London; Sarah Colbourne, from Hackney, east London; Ashan Shamrak, from north London; Hassan Ghani, from north London; Nur Choudhary, from Tower Hamlets, east London and Tauqir Sharif, from Chingford, east London.