An explosion at Macedonia's consulate offices on Thursday and the slaying of three people whose bodies were found inside may have been the work of Al Qaeda taking revenge for the killing of seven militant suspects in the Macedonian capital, police said.

Investigators found messages scrawled on a wall referring to Al Qaeda and warning against "infidels". The victims -- two men and a woman -- had their hands and feet bound and their throats slit.

The Macedonian Foreign Ministry called the assault "a professionally prepared terrorist attack" and instructed its embassies and consular offices worldwide to boost security. In a statement, the ministry also expressed "strong bitterness and deep condolences for the victims," all believed to be Pakistani.

No one claimed responsibility for the killings or the explosion.

The attack was the latest of several against foreign interests in Pakistan since military President Pervez Musharraf joined the U.S.-led war against terror after the Sept. 11 attacks in the United States.

Musharraf's stance provoked outrage among religious extremists who had found a haven in Pakistan. Pakistani security officials working closely with U.S. intelligence agencies have blamed recent attacks on Americans and other foreigners on militants linked to Al Qaeda.

In the ruins of the Macedonian consulate office, scrawled in blue ink on a wall were the words "Al Qaeda Pakistan, result of adultery" and a couplet titled "Message for Infidels" that read:

"Loyalty will be returned in loyalty.

Oppression in oppression.

We are men like you.

We will do what you will do"

The messages, written in Pakistan's Urdu language, could signal Al Qaeda's involvement or they could have been written by someone who was trying to divert investigators' attention, said Tariq Jamil, Karachi's deputy police chief.

"This could be a message or it could be an attempt to mislead the investigators," Jamil told The Associated Press.

Counterterrorism police were investigating the possibility that the slayings and subsequent explosion may have been in retaliation for the killing of seven Pakistanis in Macedonia on March 2.

Macedonian police opened fire on a van that tried to drive through a roadblock in the capital, Skopje, killing seven Pakistanis inside. Police said they found seven Kalashnikov assault rifles, several hand grenades and ammunition in the van.

Macedonian officials said the seven had planned attacks on Western embassies. One of them was identified as Ahmet Ikaz, 24, a Pakistani listed as a known criminal by Interpol.

In Macedonia, Foreign Ministry spokeswoman Santa Argirova said there was a "distinct possibility" the explosion was connected to the killing of the Pakistanis in March.

The ministry urged Pakistan to provide "firm guarantees for the personal safety of Macedonia's honorary consul, Bilal Ahmad Quereshi," who was not in the building at the time of the explosion.

Quereshi told reporters in Karachi the building should have been empty except for the night watchman.

Doctors at Karachi's Jinnah Medical Center who performed autopsies on the victims said their hands and legs were tied, their mouths gagged and their throats slit.

The weapon used was still in the body of one victim, the doctors said.

One of the dead was the night watchman, a Christian, police said. The other bodies were not immediately identified.

The explosion awoke residents, shattered windows and sent frightened neighbors running into the streets of the upscale suburb where the consulate office is located.

"I was sleeping. It was a huge explosion. I jumped the fence to escape. I didn't know what had happened," said Alladida, a housekeeper in a neighboring home.

Karachi, Pakistan's largest city, is home to 14 million people. It has also been a base for extremists groups and has been wracked by violence.

In May, a suicide bombing in Karachi killed 11 French engineers and three other people and in June a car bomb outside the U.S. consulate killed 12 Pakistanis. Wall Street Journal reporter Daniel Pearl was kidnapped in the city in January and later killed.

While U.S. special forces hunt for Usama bin Laden's Al Qaeda network in neighboring Afghanistan, U.S. intelligence believes many of the organization's top operatives have fled to Pakistan, protected by Pakistani militants.