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The Pakistani group claiming to hold kidnapped Wall Street Journal reporter Daniel Pearl extended the deadline for his execution by one day Thursday, and also warned that his abduction would be just the beginning.

"WE GIVE U 1 MORE DAY IF AMERICA WILL NOT MEET OUR DEMANDS WE WILL KILL DANIEL. THEN THIS CYCLE WILL CONTINUE AND NO AMERICAN JOUNALIST COULD ENTER PAKISTAN," said the e-mail, sent to Fox News, CNN, the BBC and three Pakistani newspapers.

The e-mail, signed with the obvious pseudonym "Mujahid Islam," went on to say that "WE HAD GIVEN OUR DEMANDS AND IF U WILL NOT THEN "WE" WILL ACT AND THE AMRIKANS WILL GET TEIR PART WHAT THEY DESERVE."

"DON'T THINK THIS WILL BE THE END," it continued. "IT IS THE BEGGINING AND IT IS A REAL WAR ON AMRIKANS.AMRIKANS WILL GET THE TASTE OF DEATH AND DESTRUCTIONS WHAT WE HAD GOT IN AFG AND PAK."

"INSHALLAH," it concluded, using the Arabic for "God willing."

A co-worker of Pearl's, Elaine Cooper, told Fox News' Rita Cosby that "letting him [Pearl] go would give these people the perfect opportunity … to represent their case."

"Harming Danny is only going to turn public opinion against them," she said.

Secretary of State Colin Powell said in Washington that "we are doing everything we can" to secure Pearl's safe release, but he ruled out meeting demands made in previous e-mails that Pakistanis held by the U.S. military in Guantanamo Bay, Cuba, be repatriated.

Powell added that he had spoken with Pakistani President Gen. Pervez Musharraf, "and I know that he is doing everything he can."

But he said "the demands that the kidnappers have placed are not demands that we can either deal with or get into a negotiation about." He insisted the detainees at Guantanamo "are being treated humanely" and "in accordance with international law."

It was impossible to be certain that Thursday's e-mail was actually sent by those who seized Pearl, 38, who vanished on Jan. 23 in Karachi. But anonymous Pakistani police sources said it appeared genuine.

The first e-mail concerning Pearl, which was signed by a previously unknown group calling itself the National Movement for the Restoration of Pakistani Sovereignty, was sent Sunday and demanded that Pakistanis captured in Afghanistan and now held at Guantanamo be sent back to Pakistan for trial before Pearl could be released.

It added that Pearl would be held in the same "inhuman" conditions as the Guantanamo prisoners, and attached several photographs of him, one of them showing him chained and with a pistol to his head, for verification.

A second e-mail, sent Wednesday, set a 24-hour deadline before Pearl would be killed, and also accused him of working for the Israeli intelligence agency, the Mossad. It also demanded that all American journalists leave Pakistan within three days or become targets, and attached more photographs of Pearl.

On Thursday, the Journal's managing editor, Paul Steiger, responded to the latest e-mail, urging the kidnappers to let Pearl go.

"The world now knows, and you seem to know, that Danny is a journalist, nothing more or less," Steiger wrote. "Journalists are, by definition, trained messengers. Danny can be your messenger. A freed Danny can explain your cause, and your beliefs, to the world.... A captive or killed Danny cannot speak for you, cannot help you or your cause."

Pearl's French-born wife, Mariane, who is several months pregnant, appealed to the kidnappers in a widely-distributed television interview to open a dialogue with her about winning her husband's freedom.

"This is completely wrong, to hold us. It's just creating more misery and that's it. Nothing can come out of there," she said.

Asked if she had a message for her husband, Mariane Pearl smiled and said: "I love you."

American boxing legend Muhammad Ali, a Muslim, invoked Allah and asked Pearl's captors to show "compassion and kindness" and release him.

"I have not lost Allah's hope in us to show compassion where none exists and to extend mercy in the most difficult of circumstances. We as Muslims must lead by example," the 60-year-old former heavyweight champion said in a statement.

Reporters Without Borders, a non-governmental organization that promotes press freedom, urged five international Islamic religious figures to appeal for Pearl's release. The appeal was made in a letter to King Mohammed IV of Morocco and religious leaders in Egypt, Pakistan, Saudi Arabia and Qatar.

Pakistani police claimed they were pursuing several leads but refused to give details. On Thursday, a source close to the investigation, speaking on condition of anonymity, said a key suspect known only as Arif was reported dead in southern Punjab province.

The source said police went to Arif's home but was told by his family that he had died a few days ago in Afghanistan. Arif was believed to have been one of Pearl's contacts, the source said.

On Wednesday, Pakistani police said they had arrested Sheik Mubarak Ali Shah Gilani, an Islamic leader with whom Pearl was trying to arrange an interview when he disappeared.

Gilani, head of the small militant group Tanzimul Fuqra, was apprehended in the northern city of Rawalpindi and transported to Karachi, police official Mazoor Mughal said.

When trying to meet Gilani, Pearl, head of the Journal's South Asia bureau and based in Bombay, was working on various stories, including possible links between Pakistani groups and Richard C. Reid, accused of trying to blow up an American Airlines plane with bombs hidden in his sneakers.

Pakistani authorities think Pearl is most likely being held by a well-known radical Muslim faction — the Harkat ul-Mujahedeen, or Movement of the Holy Warriors — which has strong links to Usama bin Laden's Al Qaeda terrorist organization.

The State Department warned U.S. citizens Wednesday to consider carefully plans to travel to Pakistan. Unconfirmed reports that Americans have been targeted for kidnapping or other terrorist actions were cited by the department in a statement to the media and to U.S. diplomats. The warning cited Pearl's disappearance.

The Associated Press contributed to this report.