The IDF stepped up its crackdown on Hamas in the West Bank on Tuesday, even as it opened the border crossings to the Gaza Strip despite continued mortar fire on the western Negev. Soldiers carried out an early-morning raid on a Nablus shopping mall that the army said was affiliated with the terrorist group, seizing the five-story mall and ordering the building's 70 shop owners to vacate the premises by mid-August. "Anyone found in this center after August 15 will be considered as working on behalf of Hamas and puts himself and his properties in danger," said an order posted in shop windows. "Profits made by stores are used to sponsor terror," a defense official said, speaking on condition of anonymity. Tuesday's raid came a day after Israeli troops swooped down on other alleged Hamas targets in Nablus. They shut down a girls' school, a medical center and two other facilities of a Hamas-affiliated charity, witnesses said. Also in Nablus, a Hamas cell that had been plotting suicide attacks inside Israel with chemical explosives was arrested in May by the Shin Bet (Israel Security Agency) and the IDF, it was released for publication on Tuesday. The cell, security officials said, had been involved in the manufacture of an explosives belt discovered in a Tel Aviv apartment on Yom Kippur last September that had been intended for use in a suicide attack in the city. The IDF arrested four members of the cell, all in their 20s and residents of Nablus. One member of the cell, Ayman Awad, had served a prison sentence in Israel for involvement in dispatching a suicide bomber several years ago. During their interrogation, the four cell members confessed to having built a laboratory in a Nablus apartment building where they had experimented with the use of various chemical materials for making a bomb, such as sulfur and nitric acid. The cell members said they had used an instructional video prepared by a senior Hamas bomb maker who had been killed in 2002 during clashes with IDF troops. Meanwhile on Tuesday, Defense Minister Ehud Barak reneged on a previous decision and agreed to an Egyptian request to open the border crossings with the Gaza Strip despite Palestinian mortar fire the day before. On Tuesday, another mortar shell slammed into a town in the western Negev. No one was injured. No Palestinian group claimed responsibility, and Barak decided to keep the crossings open. In all, 15 rockets and mortars have been fired from Gaza since the truce began. In response, Israel has kept the crossings closed about half of the time since the truce took effect on June 19. In related news, an explosion went off early Tuesday at a Hamas military training camp in Gaza, killing two of the group's members and wounding three others, both health and Hamas officials said. Hamas's military wing said in a statement that the two were killed while carrying out a "holy mission," suggesting explosives were mishandled and went off prematurely.