Avera Mengistu, an Ethiopian-Israeli citizen, has been in Gaza since September 7, according to the Coordinator of Government Activities in the Territories, or COGAT, the Israeli government division that deals with the West Bank and Gaza.
Mengistu entered the coastal enclave "by his own initiative" only weeks after the end of last summer's Gaza war, said a spokesman for COGAT administrator Yoav Mordechai. The case has been made public following the lifting of a gag order.
Mengistu's family has urged Israel and Hamas, which controls Gaza, to bring him home.
Ilan Mengistu, Avera's brother, said, "I ask the government of Israel to do everything to bring my brother home in peace. I ask the international community to become involved and to use the strength of its influence to free my brother. I turn to Hamas and ask them to take into account my brother's health situation and free him immediately."
Ilan Mengistu asked the Israeli public to show restraint at this stage as the government works to secure his brother's release.
According to Israel Radio, Avera Mengistu, 26, crossed the security fence into Gaza along the beach near his home in Ashkelon, an Israeli coastal city just north of Gaza.
COGAT would not comment on Mengistu's current condition, only saying that, "according to credible intelligence," Hamas is holding him "against his will."
'Problematic health situation'
The gag order concerning Mengistu's case was lifted following a request from the newspaper Haaretz
According to COGAT, the gag order came from the prime minister's office and allowed Israeli officials to work behind the scenes to attempt to secure Mengistu's release.
According to a senior official in the government, "quiet can serve the purposes of trying to get him back." The official said those attempts were unsuccessful.
"We felt that, if the negotiations will prevail, it's better to do it in a clandestine way," said lawmaker Yaakov Peri, who had been briefed on Mengistu's case Thursday morning. Peri is the former coordinator for the prime minister's office of Israel's prisoners of war and those missing in action.
Peri said Mengistu had gone missing several times before. Mengistu "was not recruited to the army, probably because of mental reasons," he said.
Ilan Mengistu said his brother has a "problematic health situation" but did not elaborate.
In the days after Mengistu apparently crossed into Gaza, Hamas announced it was holding Israeli prisoners, but there are conflicting reports as to whether Mengistu is still in Gaza or has crossed into Egypt.
"A couple weeks after that, (Hamas) told us, 'Yes, we know that he was in Gaza, but he was in Gaza and left to Egypt.' We know that's not true, and he is still probably held by Hamas in the Gaza Strip," Peri said.
"We see Hamas as responsible for bringing him back safely."
An Israeli Arab is also being held in Gaza, according to COGAT, but it wouldn't give any more details.
President: This is a humanitarian issue
Mengistu's fate has come up in meetings between Ethiopian-Israeli leadership and the prime minister's office, said Fentahun Assefa-Dawit, executive director of Tebeka
, an Ethiopian-Israeli advocacy organization.
Ethiopian-Israeli leaders were told that the prime minister's office was working to secure Mengistu's release, according to Assefa-Dawit.
"We as an organization will try to think about what's the best way to tackle this," Assefa-Dawit says, "(to see) if we can assist in any way from our end to facilitate or do anything in bringing Avera back home."
Prime Minister Benjamin Netanyahu said in a statement Thursday that the government was working to secure the return of Mengistu and a second Israeli and was in touch with their families.
"We hold Hamas responsible for their welfare," he said, adding that Israel had "spared no effort" in seeking Mengistu's return.
"I expect the international community, which expresses its concern over the humanitarian situation in Gaza, to issue a clear call for these citizens to be released and to see to their return," he said.
Israeli President Reuven Rivlin has also been following the case ever since Mengistu went missing.
"This is a humanitarian issue, and we expect those holding him to behave accordingly and return him in good health," Rivlin said in a statement.
"According to the information which has arisen, Mengistu is being held by Hamas in the Gaza Strip. We will continue to make every effort in order to bring an end to this incident, as soon as possible."