Mohammed Nassar has not seen his son in 15 years.

On March 7, 2002, during the Second Intifada, or Palestinian uprising, he says his son Shadi "left the house and never came back".

Shadi had gone to carry out a suicide bombing at the entrance to the illegal Israeli settlement of Ariel in the occupied West Bank, killing himself and wounding 15 Israelis.

"We had no idea that he was planning to carry out an attack," Nassar says.

"If I had any suspicions, I would have beat him up, and thrown him at home.

"A father losing his son? How am I supposed to feel?"

Fifteen years later, Israel continues to refuse to return Shadi's body, or what's left of it, to his family for burial.

The youth's remains are said to be in Israel's infamous "cemeteries of numbers", along with hundreds of other Palestinians. In a recent report published last month, Palestinian rights groups estimated there are at least 249 Palestinians, including Shadi, buried in cemeteries across Israel in closed military zones, and nine others in freezers in Tel Aviv.

The cemeteries are made up of mass graves marked with numbers rather than names, and some of the bodies have been there since the 1967 war.

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"We're always nervous ... even though it's been 15 years," Nassar tells Al Jazeera from his home in Nablus in the occupied West Bank, continuing: "Sometimes, we get doubts about whether he's really dead, because we haven't buried him ourselves."

"It's been extremely difficult for us. Only God knows how we've managed to get through these years.

"This is a form of psychological torture. Every day is a painful day. We keep getting ideas that he may still be alive."

Shadi's attack came in the context of the Palestinian uprising against Israel's occupation of the Palestinian territories.

It began with the failure of the Oslo Accords in 1993 to create a Palestinian state and Israeli politician Ariel Sharon's visit to al-Aqsa Mosque compound.

Violence erupted after the then Likud leader's visit, unleashing the Second Intifada between 2000 and 2005.

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The attacks by Palestinians - largely viewed by Palestinians as legitimate armed resistance to a violent, 50-year occupation - led to the deaths of some 3,000 Palestinians and 1,000 Israelis.

The majority of the bodies currently being held by Israel date back to that period.

Abu Issam, father of Abdel Basset Odeh, who killed 22 Israelis in a suicide bombing attack, and whose body remains with Israel, agrees that the practice of withholding bodies is "a form of torture".

"It's difficult after you raise a child for 25 years to deal with this," Abu Issam says.

"He was exposed to a lot at from young age ... many of his friends and family members were killed. He was imprisoned in 1994 for five months when he was just 12.

"If they were in the cemetery of numbers, we get back the remains of their bodies in a black bag – some bones, some soil, and maybe their clothes". Salwa Hammad, lawyer

"The things they see at a young age force them to seek revenge for what they [Israel] inflicts on us," Abu Issam tells Al Jazeera from the occupied West Bank city of Tulkarem.

In a recent video that went viral on social media, the friends of a Palestinian who was killed by Israeli soldiers in Jerusalem smuggled his body out of a hospital and buried him before Israeli forces, who were raiding the hospital, could seize him.

The practice has long pained Palestinian families, who sometimes have to wait decades before receiving the bodies of their loved ones.

It is widely known that Israel employs the practice as a tactic for leverage in negotiations. In 2012, Israel released the bodies of 90 Palestinians in a gesture for reviving peace talks between Israeli and Palestinian officials. And, between 2013 and 2014, some 27 bodies were returned.

On Friday, Israel returned the bodies of four Palestinians killed by Israeli forces in July.

Collective punishment

The practice is a violation of both Israeli domestic and international law.

The Geneva Conventions state that the parties of an armed conflict must bury the deceased in an honourable way, "if possible according to the rites of the religion to which they belonged and that their graves are respected, properly maintained, and marked in such a way that they can always be recognised".

It is also illegal under Israeli law. On July 25 of this year, the Israeli Supreme Court issued a ruling on a petition by Adalah, a Palestinian-run legal centre in Israel, stating that the Israeli police have no authority to withhold bodies, according to Adalah lawyer Mohammed Bassam.

Likewise, in its 2016 review of Israel's compliance with the United Nations Convention against Torture, the UN Committee Against Torture expressed concern regarding Israel's use of this practice.

The committee urged Israel: "to return the bodies of the Palestinians that have not yet been returned to their relatives as soon as possible so they can be buried in accordance with their traditions and religious customs, and to avoid that similar situations are repeated in the future".

To help families retrieve the bodies of their loved ones, the Jerusalem Legal Aid and Human Rights Center (JLAC) has launched a national campaign to force Israeli courts and authorities to return the bodies.

They document the number of bodies in two ways; when a family member notifies them, or through the political organisations and movements that the Palestinians belonged to.

In 2010, JLAC managed to retrieve the body of Mashour al-Arouri, after 34 years of being in Israeli custody. In 2011, they released the body of Hafez Abu Zant, after 35 years.

Salwa Hammad, lawyer and campaign coordinator, estimates there are six cemeteries of numbers. A court decision is required for Israeli authorities to move the body from a freezer to a cemetery.

"They call the family in for interrogation and show them pictures of the body. They do a DNA test - they take a sample from the substance in the knee bones, as well as saliva samples from a first-degree relative and they see if the samples match," Hammad told Al Jazeera.

"If they were in the cemetery of numbers, we get back the remains of their bodies in a black bag - some bones, some soil, and maybe their clothes".

Hammad explains that the practice places a huge psychological burden on the family.

"The main reason that they do this is to collectively punish the family of the martyr. I can't tell you how emotionally affected the family gets," she says, continuing: "We get calls regularly from families telling us that they heard rumours from some one that was released from prison, saying that they heard information about their son."

"As long as they cannot see the body of their son, they have hope that he may still be alive - that he may be in prison."

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Mervat Nahhal from the Al Mezan rights centre in Gaza, says that Israel began employing this tactic in a systematic way starting from the 2000s.

"Israel does not give death certificates for the bodies it is withholding and it does not reveal the names of the martyrs that it is withholding. That's the problem with Israel - it does not abide by international law," Nahhal says.

Still, the families vow to continue the fight to retrieve their sons' bodies.

"We're doing everything in our power. We're always hopeful to get him back," Nassar said.