LONDON – Prime Minister Blair appealed to Muslim leaders yesterday to combat the “twisted logic” of terrorism and offered to help them counter extremism with reason. In a show of vigilance, police deployed dogs for the first time to sniff out explosives on London’s Underground.

The developments in London came as police in Pakistan focused their investigation on the eastern city of Lahore, rounding up seven Islamic militants linked to Al Qaeda and suspected of having ties with the British suicide bombers. And in Cairo, the Egyptian government said a detained chemist wanted for questioning by Britain had no links to the attacks or to Al Qaeda.

British officials were also examining whether bomber Jermaine Lindsay, a Jamaican-born Briton, used perfume bottles to make his bomb deadlier. The explosions on three subways and a double-decker bus in London killed at least 56 people.

Asked about the inquiry into the July 7 attacks by Lindsay and three other bombers, the chief of the Metropolitan police, Ian Blair, said key questions remain, including: “Who is the chemist? Who are the people who trained them? Who facilitated their trip to Pakistan?”

“Whoever is doing that is still out there,” the police chief told a meeting of Christians, Muslims, and Jews that was attended by the Associated Press.

Fearful of another attack since accomplices of the bombers or a mastermind may be at large, British Transport Police dispatched dogs yesterday to search for explosives on the London Underground.

Dogs have been used before on the train that connects Heathrow airport to the capital, but police said this was the first time they were being sent into the subway system.

Mr. Blair met with two dozen representatives of the Muslim community to discuss anti-terror legislation the government plans to introduce by year’s end. The leaders fear the laws target their community.

“It’s fair the government should ask itself whether policies such as those involving the Iraq war have contributed to this,” the head of the Muslim College, Zaki Badawi, said. “We need a partnership between government and Muslims to show people they are not being ignored and that their concerns will be heard.”

The prime minister denied any link between the Iraq war, which was opposed by many Britons, and the attacks in London. He insisted that terrorists will always find an excuse to kill – and promised action to uproot their “perverted ideology.”

“Of course these terrorists will use Iraq as an excuse. They will use Afghanistan,” Mr. Blair said at a news conference after meeting with President Karzai of Afghanistan.

“There is no link,” Mr. Karzai said. “They are simply merchants of death.”

The rugged border area between Afghanistan and Pakistan is thought to be a hiding place for Al Qaeda and Islamic extremists.

In Pakistan, police said they were holding seven Islamic militants with possible links to the London suicide bombers.

Three of the London suspects – Hasib Hussain, 18, Mohammed Sidique Khan, 30, and Shahzad Tanweer, 22, all Britons of Pakistani descent – traveled to Karachi in southern Pakistan last year.

Pakistani security officials said they believe Tanweer spent a few days at a religious school in Lahore, an eastern city where many militant groups have clandestine operations.

Lahore is the capital of Punjab province, where five of the detained men were picked up in recent days, Pakistani officials said. The two others were caught overnight in Pakistan’s southern Sindh province.

Police officials in Pakistan said the seven men were from two outlawed terrorist groups, Lashkar-e-Jhangvi and Jaish-e-Mohammed. Both are linked to Al Qaeda, and some of their supporters have been arrested for trying to assassinate President Musharraf.

Investigators are trying to establish whether the bombers received training, encouragement, or other aid from extremists in Pakistan, or even if the plot was hatched there.

Pakistan is not the only foreign country Khan apparently visited. In Jerusalem, an Israeli security official confirmed yesterday that Khan went to Israel in 2003.

The Israeli official, who spoke on condition of anonymity because of the nature of his work, did not elaborate. But the Israeli daily Maariv reported Sunday that Israeli security officials believe Khan was among the planners of an April 30, 2003 suicide bombing on a Tel Aviv bar that killed three people and wounded 60.

Another focus of the investigation has been an Egyptian chemist, Magdy el-Nashar, who obtained a doctorate from Leeds University this year. Authorities in Cairo detained Mr. el-Nashar last week at the request of the British government, which suspects he has links to some of the four bombers.

Egyptian government spokesman Magdy Rady said yesterday an Interior Ministry report “made clear that there is no link between Egyptian chemist Magdy el-Nashar with Al Qaeda or the bombings.”

Scotland Yard declined to comment.

Investigators are trying to determine what material was used in the four bombs.

Yesterday, a statement in the name of a group that claimed responsibility for the London bombings threatened to launch “a bloody war” on the capitals of European countries that do not remove their troops from Iraq within a month. The authenticity of the statement – which surfaced on an Islamic Web site – could not be verified.