A shattered windshield as Palestinians inspect a car after a ramming attack on two Israeli soldiers at the Palestinian refugee camp of Qalandia, north of Jerusalem, on Dec. 16. (Atef Safadi/European Pressphoto Agency)

A CAR carrying an Israeli family was struck by gunfire as it traveled toward a West Bank settlement Oct. 1. When the driver was hit and the vehicle stopped, the gunmen approached and coldly murdered Eitam and Naama Henkin as their four children watched from the back seat. Soon after, Israeli security forces arrested what they said was a five-member Hamas cell from the town of Nablus that allegedly had carried out the assault.

Now the Palestinian Authority, the secular government that controls Nablus and other major West Bank towns, has made its own arrest — of a respected journalist. Salim Sweidan, a board member of the Maan News Network, was picked up by security forces from Nablus on Jan. 7. A prosecutor soon charged him with “slandering public authorities, making up and publishing false news, violating Palestinian press laws . . . publishing material inciting hatred and violence, and publishing material that can harm national unity.”

Mr. Sweidan, who was released on bail this week, is not a Hamas member or supporter; on the contrary, he is close to the secular Fatah movement of Palestinian Authority President Mahmoud Abbas. His offense was to publish an article on a website reporting that Palestinian security services provided Israel with information leading to the arrest of the Hamas cell.

That this has been cause for his prosecution is worrying evidence of two trends gathering momentum in the West Bank: the breakdown of the rule of law under the Palestinian Authority and the growing reluctance of its security forces and their leaders to maintain a relationship with Israel that has been vital to preserving a relative calm in the territory.

Mr. Abbas has declared that no Palestinian journalist should be arrested for what he writes. As fellow Palestinian journalist Daoud Kuttab pointed out, Mr. Sweidan’s detention clearly violated the Palestinian Authority’s law. So did a subsequent demand by Nablus authorities that a website that reproduced his article publish an apology. Yet Nablus security forces carried out the operation without regard for the constitution or the 80-year-old Mr. Abbas — who himself remains in office seven years after his term expired.

Of equal concern is the security forces’ zeal not to be linked to Israel’s arrest of militants who carried out a brutal murder of civilians. For months, Palestinians have been carrying out uncoordinated but frequent attacks on Israelis. Mr. Abbas and other Palestinian leaders say they oppose the violence but at times have appeared to encourage it with inciting statements, and with visits to the families of Palestinian assailants killed by Israeli security forces. Some Palestinian leaders are pushing for a decision to end security cooperation between Palestinian security forces and Israel. That step would almost certainly lead to the suspension of U.S. aid to the Palestinian Authority and its collapse.

In short, the arrest of the Palestinian journalist is not just an example of the disregard for press freedom by Palestinian leaders. It is also another sign of an approaching crisis that Israel and the United States should be trying to head off.