KARACHI, PAKISTAN -- An explosion at Macedonia's consulate offices 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 Skopje, the Macedonian capital, police said yesterday.

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

The Macedonian Foreign Ministry called the assault "a professionally prepared terrorist attack" and instructed its embassies and consular offices worldwide to reinforce security. 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 terrorism after the Sept. 11 attacks.

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

Then, Macedonian police opened fire on a van that tried to drive through a roadblock, 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" that the explosion was connected to the Skopje killings.