GAZA CITY, Gaza Strip (AP) — Authorities in the Gaza Strip on Monday announced the first coronavirus cases spread through the community, raising fears of a potentially devastating outbreak in the impoverished Palestinian territory blockaded by Israel and Egypt.

Until now, all the cases reported in Gaza were linked to quarantine facilities for residents returning from abroad.

The Health Ministry said four people from the same family have tested positive for the virus in central Gaza and investigations were underway to track the source of the infection. A full lockdown was imposed on the al-Maghazi refugee camp, where the family lives.

The ministry said a woman from Gaza who was allowed to travel to Jerusalem for medical treatment tested positive. Health workers in Gaza then tested her family members, revealing the four cases.

The Islamic militant Hamas group, which rules Gaza, announced a 48-hour curfew in the entire territory, closing businesses, schools, mosques and cafes.

Hamas seized control from rival Palestinian forces in 2007. In response, Egypt and Israel imposed a crippling blockade on the territory, which is home to some 2 million Palestinians.

Israel says the blockade is needed to keep Hamas from importing and manufacturing arms. Hamas and Israel have fought three wars and countless skirmishes in the last 13 years.

Tensions have risen in recent weeks as Palestinian militants have launched incendiary balloons and rockets across the Gaza frontier. Israel has responded with airstrikes on targets linked to Hamas. There have been no deaths or serious injuries on either side.

The recent attacks were coordinated by Hamas to pressure Israel to ease the blockade. Instead, Israel has closed Gaza's sole commercial crossing and its fishing zone. The territory's only power plant was forced to shut down for lack of fuel, leaving most Gazans with just four hours of electricity a day.

Gaza's heath infrastructure has been hollowed out by years of war and isolation, and would be ill-equipped to cope with a major outbreak. Gaza's health facilities only have around 100 ventilators, more than half of which are already being used.

The discovery of the local cases comes despite months of strict efforts taken to prevent community transmission.

Since March, Hamas has imposed mandatory isolation for 21 days at designated quarantine centers for all those returning by way of Israel and Egypt. Authorities have detected 109 cases in the quarantine facilities since March, and 72 of them have recovered. The only fatality in Gaza was a woman who had underlying health problems.

The coronavirus causes mild to moderate symptoms in most patients, who recover within a few weeks. But it is highly contagious and can cause severe illness or death, particularly in older patients or those with medical conditions.

The apparent success in keeping the virus out had led to the lifting of virtually all measures to contain it. Schools recently reopened, and wedding halls and mosques have been operating normally for more than two months.