Mexican authorities have arrested a former intelligence agent suspected of involvement in the 1994 assassination of presidential favourite Luis Donaldo Colosio, a case that stunned the nation and remains unresolved.

Judicial sources told AFP that Jorge Antonio Sanchez Ortega was detained on Saturday in Tijuana, Baja California, and brought before a judge.

Colosio, the Institutional Revolutionary Party candidate, was shot during a campaign rally in Tijuana.

His murder on March 23, 1994 is seen as a pivotal moment in Mexican political history.

Weeks earlier Colosio had delivered a speech criticizing the corrupt old guard of the ruling party.

As with the assassination of US President John F. Kennedy, Colosio’s murder unleashed a storm of conspiracy theories about who was behind it.

Although Mario Aburto Martinez was convicted as the sole perpetrator and remains in prison, the identity of those who may have ordered the killing is still unknown.

Sanchez Ortega was briefly arrested at the time but released the next day. According to reports, he was employed then as an agent for the Centre of Investigation and National Security – back then Mexico’s equivalent of the US Central Intelligence Agency.

Authorities have not confirmed whether new evidence has emerged.