The killings began on September 16, 1982 and lasted for two days until the 18th of the same month, during the Israeli invasion of Beirut.
The documents, published in Hebrew, translated by Israeli activist Roni Barkan, and excerpts from the Haaretz newspaper, reveal chapters of the history of the relationship between the Israeli side and the "Phalange" militia and the Lebanese "Ahrar".
The documents confirm that Israel's relationship with Lebanese political parties and figures began in the fifties.
It talks about a plane sent by the regime of the Shah of Iran, Muhammad Reza Pahlavi, to ship weapons from Israel to Lebanon, in support of the then Lebanese president, Camille Chamoun in 1958.
What happened in Beirut in 1982?
Israeli forces invaded Lebanon on June 6, 1982, after the attempted assassination of the Israeli ambassador in London, Shlomo Argov. He called the operation "Peace for the Galilee", and announced that its goal was to eliminate the activities of Palestinian factions in Lebanon.
After 90 days of bombarding and besieging the capital, Beirut, a settlement agreement came out that stipulates the exit of the leaders of the Palestine Liberation Organization and its fighters from Lebanon.
Meanwhile, Bashir Gemayel, the leader of the Lebanese Forces - initially established as a military arm of the Kataeb Party - was elected President of the Republic. He was assassinated three weeks later, on 14 September.
The day after the assassination, Israeli forces entered the capital, Beirut. On September 16, killings began in the Sabra and Shatila camps.
An Israeli commission of inquiry indicated that between 700 and 800 people were killed during the massacre, but other sources say that the number of victims is much higher.
The American envoy to the region at the time, Philip Habib, had given guarantees about the security and safety of unarmed Palestinian civilians, who had not left Lebanon.
What came in the Mossad report?
The first Israeli shipment of weapons to Lebanese militias was launched in mid-November 1975.
The first shipments of weapons sent by sea were delivered to the fighters of the "Patriotic Liberal Party", founded by former President Camille Chamoun. (In the documents, they are called the Shimonites).
After the success of the arrival of the first shipment, shipments continued, and weapons were sometimes sent to three parties, the "Shamoonites", Bashir Gemayel and Amin Gemayel (brother of Bashir, son of the founder of the Phalange Party, Pierre Gemayel).
The first threads of the relationship between the Israeli and Lebanese sides began in the fifties, regarding securing protection for Christians in Lebanon.
Camille Chamoun was president of the republic, and his rule was under threat in 1958, so an agreement was made with the Shah's government to send a plane to transport weapons from Israel to Lebanon in support of him. And then sent several shipments.
When the situation in Lebanon deteriorated in 1975, Camille Chamoun was the first to contact the Israeli side asking for help and arms.
The Israeli leadership agreed to supply the Lebanese with light weapons in the beginning in return for paying the cost, or the price of re-operating some of the weapons seized in the "October War of 1973".
The relationship with the Lebanese side was limited to the supply of weapons, and the two sides did not enter into any deep or far-reaching political discussions, until the summer of 1976.
A delegation made up of the Mossad and army intelligence visited Lebanon secretly, and stayed for three days to assess the situation of their allies in the war.
The delegation visited the headquarters of the Kataeb and "Shamoonite" leaders, and also met with Tony Franjieh (son of former President Suleiman Franjieh). He also had lunch at Al-Jamil's house in the village, where they met Bashir Al-Gamil.
Discussions with the delegation centered on Israel's policy to help Christians in Lebanon.
After that, he began sending individuals or officials from the Lebanese militias to Israel to receive training courses in various fields.
In January 1982 (five months before the invasion), Ariel Sharon visited Lebanon and met with the leadership of the "Ahrar" and the "Phalange".
What is the new Israeli documents?
This may be the first time that the Mossad has officially revealed relations between Israel and Lebanese officials dating back to the 1950s - that is, before the years of the civil war in the 1970s - and in particular its support for President Camille Chamoun in 1958.
The documents also reveal the Iranian role during the reign of Shah Mohammad Reza Pahlavi, by ensuring the transfer of weapons from Israel to the Lebanese side.
The documents also published a detailed list of weapons, ammunition and military equipment, which Camille Chamoun requested from the Israelis during the civil war and expressed his willingness to pay for them.
The documents stated that Ariel Sharon (then Minister of Defense) told Pierre Gemayel in January 1982, "We are going to a full-scale war, which will bring about a change in Lebanese-Israeli relations."
The documents also stated that the Israeli preparations for the Lebanon War began in the middle of 1981, and accelerated at the end of the year. They visited the field and saw for themselves where they would fight.