Major Roberto Samcam, 66, was gunned down at his apartment building in San Jose, reportedly by men pretending to deliver a package.

"It was something we did not expect, we could not have imagined it," Samantha Jiron, Samcam's adoptive daughter, told AFP from her home in Madrid, adding that her father was shot eight times.

"Roberto was a powerful voice" who "directly denounced the dictatorship" of Ortega, Samcam's wife Claudia Vargas told reporters in San Jose as she fought back tears.

His job, she said, was to "expose human rights violations" in his homeland.

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

"It is an act of cowardice and criminal political revenge by the dictatorship of Nicaragua," the country's former ambassador to the Organization of American States, Arturo McFields, who lives in exile in the United States, wrote on X.

The US State Department's Bureau of Western Hemisphere Affairs said on X that it was "shocked" by Samcam's murder and offered Costa Rica help in "holding the assassins and those behind them accountable."

Samcam, who was a political analyst, 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 over 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.

While the motive of that attack was the object of much speculation, Samcam's killing fueled suspicion among Nicaraguans that it may have been linked to his political activities.

The Nicaraguan news site Confidencial reported that Samcam's killers fled by motorbike.

- 'Night of long knives' -

Former Costa Rican president Luis Guillermo Solis called Samcam's murder "for his frontal opposition to the Ortega and Murillo dictatorship" an "outrageous and extremely serious act."

"I feel that Daniel Ortega and Rosario Murillo are initiating a 'Night of the Long Knives'... due to the regime's weakening," Dora Maria Tellez, a former associate of Ortega turned critic, said from Spain, where she too is in exile.

The "Night of the Long Knives" was a bloody purge of rivals ordered by Nazi leader Adolf Hitler in 1934.

"They resort to the execution of a retired ex-military officer, whom they believe has a voice that resonates within the ranks of the army," Tellez told the Nicaraguan news outlet 100% Noticias.

Ortega, now 79, first served as president from 1985 to 1990 as a former guerrilla hero who had helped oust a brutal US-backed regime.

Returning to power in 2007, he became ever more authoritarian, according to observers, jailing hundreds of opponents, real and perceived, in recent years.

Ortega's government has shut down more than 5,000 NGOs since the 2018 mass protests that he consider a US-backed coup attempt.

Thousands of Nicaraguans have fled into exile, and the regime is under US and EU sanctions.

Most independent and opposition media operate from abroad.