Tension has been rising in Jerusalem since the summer. And in Jerusalem, tension, coupled with the absence of any light on the political horizon, tends to escalate into violence.
It has been fed by the fact that once more the conflict between Israelis and Palestinians has been left to fester. An attempt by the Americans to revive a peace process failed, despite energetic diplomacy from the US Secretary of State John Kerry, reports BBC.
The two sides are further apart than ever. Their conflict used to be, at root, about the possession of land. But since Israel captured the West Bank, including East Jerusalem, in 1967 it has become more defined by religion.
Perhaps that was why the Palestinians chose a synagogue for the attack that killed the four Jewish worshippers. There have been other attacks on Israelis in recent months by Palestinians, one of which killed a baby.
Many Palestinians believe Israel is preparing to allow Jews to pray in the compound of the al-Aqsa mosque in Jerusalem, the third holiest site for Muslims after Mecca and Medina.
The Israeli government has denied that emphatically. But Palestinians listen to calls from hard right-wing Jewish nationalists and believe it might happen.
Some Israeli right wingers say that they have a right to pray inside the compound as it was the site of the Jewish Temple until it was destroyed by the Romans some 2,000 years ago, and remains the holiest site in Judaism.
The Palestinian President Mahmoud Abbas has called for Palestinians to defend al-Aqsa. For Palestinians that sounds reasonable. The Israeli government has condemned it as incitement to terrorism.
President Abbas commands some respect among Palestinians, but he is not very popular.
The failure of the Fatah movement, which he leads, to deliver an independent state has caused bitterness and cynicism among many Palestinians.
Calls to defend al-Aqsa are the kinds of things Palestinians want to hear from their leaders.
President Abbas, who has said many times that he is against the use of violence, has been overshadowed for many Palestinians in recent years by Hamas, the Islamist rivals of Fatah in the Palestinian national movement. In the last five years Hamas has gone to war three times.
Palestinians are also angry about the continued growth of Jewish settlements in East Jerusalem. The big settlements in occupied land in East Jerusalem were built on largely open ground. But now the emphasis is on settling Jewish families in areas that are otherwise populated by Palestinians.
The proximity of the two sides, and the feeling that Palestinians have that their land is being taken by armed settlers, leads to trouble.
A particular flashpoint is Silwan, near the walled old city, which settlers have renamed City of David.
There is almost daily violence there between local Palestinians and the expanding community of Jewish settlers, who are protected by armed guards paid for by the Israeli government.
The two Palestinian killers this morning are from the district next to Silwan.
Both Palestinians and Israelis are now talking about a third Palestinian uprising – or intifada. It’s too early to say one has started. But in the absence of political action to stop the violence escalating, another intifada is a distinct possibility.