Mahmud regrets the day he took cash from an Israeli soldier for his wedding. It cost him a life sentence in a Gaza jail for collaboration with Israel.
Some of them let themselves be bought for a pittance – receiving only a few hundred shekels (about $80) – in exchange for betraying their countrymen, Hamas says.
For Mahmud, financial problems paved the slippery slope to jail.
"I was walking near my home in northern Gaza, near the border with Israel, when I was stopped by the special forces and questioned," he says, crouching in the corner of his cell.
"The intelligence officer asked me what I was doing there and as we were talking, I told him I was depressed because there was only a month left until my wedding and I didn't have enough money to pay for it," he explains.
"He told me: Work for us and we will give you the money.'
"Then he gave me NIS 1,000 ($275) and an Israeli SIM card and told me they would ask me a few questions, and that in exchange for answers, they would pay me the same amount every time which would help me pay for my wedding," says Mahmud, his eyes fixed firmly on the ground.
As planned, the Israeli officer called him a month before his wedding and asked him to keep an eye on the movements of two of his neighbors who belong to Hamas's armed wing, the Izz al-Din al-Qassam Brigades.
"I tried to lie to him but he already knew everything," he says with a shrug.
Mahmud says he worked for the Israelis for around a year before being arrested in 2008 after being exposed by one of his cousins, in whom he had confided, who works for the Hamas authorities.
"I confessed everything," he says.
Fellow inmate Shadi, who has already served one year of his seven-year sentence, says he was recruited in similar circumstances after being arrested by Israeli troops near the border.
"I agreed because I was frightened and because I needed money," the 21-year-old told AFP, saying an intelligence officer had given him NIS 500 ($135) and an Israeli SIM (mobile telephone) card.
"He asked for information about places from which they fire rockets (into Israel) and the names of those responsible for the firing," says Shadi, who used to be a Hamas activist.
"I had a lot of friends in Hamas and in the al-Qassam Brigades. The officer asked me about them and he always knew if I was lying."
32 alleged informers executed
One day as he was heading for the border to meet his Israeli contact, he run into a group of militants en route to fire rockets into Israel. They smelled a snitch and hauled him before the Hamas commander in the area.
"I told him everything," he says.
Prison director Nasser Suleiman, whose establishment is referred to as a reeducation center, says around 40 Palestinians are being held for collaborating with Israel.
"We realized that the economic issue, caused by the blockade, was the main means of pressure on potential collaborators," said Ihab al-Gussein, interior ministry spokesman, referring to the blockade imposed by Israel in 2006 which has been eased in the last nine months.
Financial difficulties or not, Hamas has vowed to ruthlessly track down anyone believed to be collaborating with the Israelis.
In December, a Hamas military court convicted three men on collaboration charges, sentencing one to death and two more to prison terms.
And last April, the Hamas authorities executed two alleged collaborators in the first executions to be carried out since the Islamist movement seized power in June 2007.
Israeli security forces routinely canvass Palestinian informers in the occupied territories in their quest for intelligence to thwart attacks and hunt down top militants.
Human Rights Watch says Hamas executed at least 32 alleged informers and political opponents and maimed dozens of others both during and after the 22-day Israeli assault on Gaza which ended in January 2009.
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