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.

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

"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.

The head of Costa Rica's judicial police, Randall Zuniga, said that the attackers took advantage of the fact that Samcam's apartment building was unguarded in the mornings.

The gunman "called out to... Roberto," who "approached without knowing" the danger, Zuniga said.

"When he was within striking range, the individual began shooting at him and hit him at least eight times," he told reporters.

The Nicaraguan news site Confidencial reported that the killers fled the scene by motorbike.

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."

Nicaragua's former ambassador to the Organization of American States, Arturo McFields, who lives in exile in the United States, called the killing "an act of cowardice and criminal political revenge by the dictatorship of Nicaragua."

"The manner of the crime indicates political motives. This is very serious," Nicaraguan writer Gioconda Belli, exiled in Spain, stated on X.

Neither Ortega nor his government commented on the case.

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 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.

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

- '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 non-governmental organizations since the 2018 mass protests that he considered 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.

Pro-government media in Nicaragua did not report on Samcam's killing.