Police investigated on Monday the murder of a top aide to Mexico's leading opposition presidential candidate following accusations the killing was politically motivated.

Francisco Xavier Ovando, election coordinator for candidate Cuauhtemoc Cardenas, was found dead in his car late Saturday night in downtown Mexico City along with his aide Roman Gil. Both men had been shot in the head.Mexicans vote on Wednesday in a race where ruling Institutional Revolutionary Party (RI) candidate Carlos Salinas Gortari is facing an unusually strong challenge from the leftist Cardenas and right-wing candidate Manuel Clouthier.

The Federal District Attorney General's office released a statement Monday night saying police were conducting a full investigation into the murders.

"We can't say yet it was a political killing but there is every indication it was," Saul Figueroa, Cardenas' personal secretary, told Reuters.

Ovando was shot four times in the head at close range while Gil received one bullet in his left ear.

Cardenas told reporters on Monday he had sent a letter to President Miguel de la Madrid and Federal District Attorney General Renato Sales Gasque demanding prompt action on the crime.

The candidate for the National Democratic Front (DN) said he expressed his concern in a Monday meeting with Interior Minister Manuel Barlett that the killing could mean the start of a wave of election-related violence.

Ovando, 47, was responsible for setting up an FDN mechanism for monitoring the vote throughout Mexico in an effort to prevent fraud.

Both Cardenas and Clouthier have warned throughout their campaigns that the PRI is planning a massive fraud in order to ensure a comfortable victory for Salinas de Gortari. The PRI has not lost the presidency since its foundation in 1929.

The official party, however, has consistently denied the fraud charges.