Retired Nicaraguan army major Roberto Samcam, 66, was gunned down at his apartment building in San Jose on June 19.

Nicaraguan rights groups and exiled dissidents blamed the government of Ortega and his wife and co-president Rosario Murillo.

Costa Rican police carried out several raids since Thursday in an area of San Jose where many Nicaraguan migrants live, as well as a town northwest of the capital, arresting four people in all.

"We have an individual with the last name Carvajal, 20 years old, who is believed to have committed the homicide," Randall Zuniga, director of the Judicial Investigation Agency, said in a video sent to journalists Friday.

Carvajal was allegedly accompanied at the crime scene by two of the other detainees.

Police will now seek to determine "whether we are indeed dealing with a political incursion from another government," added Zuniga.

Police were still looking for a fifth suspect.

Zuniga said the crime was committed by "inexperienced individuals" hired from poor communities.

Samcam had spoken out frequently against the government in Managua, which he fled in 2018 to live with his wife in Costa Rica.

That year, protests against Ortega's government were violently repressed, resulting in more than 300 deaths, according to the UN.

In January last year, another Nicaraguan opposition activist living in Costa Rica, Joao Maldonado, was shot while driving with his girlfriend in San Jose. Both were seriously wounded.

Maldonado had also survived an attempt on his life in Costa Rica in 2021.

After Samcam's killing, a UN expert group urged countries hosting Nicaraguan exiles to offer them stronger protection, saying "nowhere in the world seems to be safe for Nicaraguans opposed to the government."

Ortega, who turns 80 in November, is increasingly delegating tasks to his septuagenarian wife, Murillo, who has a reputation for ruthlessness.

Observers have said that a recent wave of arrests and deaths of jailed opposition figures signal a new "era of repression" in Nicaragua as Murillo prepares to take over.

Thousands of Nicaraguans have fled into exile as the government has become increasingly authoritarian, jailing hundreds of opponents, real and perceived, according to observers.

Managua is under US and EU sanctions.