Havana, Cuba (CNN) Some of the 21 US diplomats believed to have been impacted by mysterious acoustic attacks in Cuba were targeted multiple times, CNN has learned from a senior US official.

There were nearly 50 attacks in total, the official said.

The incidents have challenged the US government's assessment that Cuba is a safe country for US diplomats and their families and threatened the future of the newly reopened embassy.

Despite the often-empty supermarkets and antagonistic relations with the communist-run government, Cuba for years offered US diplomats a rare benefit: It was safe.

Unlike in many other countries, in Cuba, US Embassy employees didn't have to worry much about terrorist attacks, kidnapping or even petty crime. The Cuban government's tight control over the island made Havana one of the safest cities in the world.

Diplomats -- especially those Cuba suspected of being spies -- might suffer harassment at the hands of the powerful state security apparatus, but there were established lines neither of the Cold War adversaries would cross.

But starting early this year, US diplomats heading to the island to begin their postings were quietly warned they could face a mysterious threat that was causing American Foreign Service officers to fall ill, some with long-lasting symptoms.

Investigators haven't determined the cause of the incidents, but US officials told CNN they are convinced someone has targeted American diplomats in Havana with a sophisticated device never deployed before, at least not against US personnel.

At the United Nations on Friday, Cuban Foreign Minister Bruno Rodríguez Parrilla denied that Cuba was involved in attacks on diplomats and said the United States was politicizing the incidents.

But seven months after complaints to officials and assurances from Cuban President Raul Castro that the incidents would be investigated, US officials are frustrated by the lack of progress and may scale back the embassy to limit the number of people who risk exposure.

Options include sending families and nonessential staff back to the United States to a full-on shuttering of the embassy , three senior US officials told CNN.

"We have to consider it. We thought we had corralled this, and then the two cases in August took place," a senior US official said. "It is not as if the attacks address individual personnel officers. Our personnel is broadly at risk. So we have to consider next steps because we need to protect our people."

Setback in relations possible

If the United States were to recall diplomats, it would be a devastating setback to US-Cuban relations and come at a crucial moment as Castro prepares to step down as President in February and Washington needs eyes and ears on the ground.

Ties between the countries were severed in 1961 shortly after Fidel Castro took power. As confrontation between the two nations loomed, US diplomats hurriedly lowered the American flag at the embassy and boarded a ferry to sail across the Straits of Florida.

In 1977, the United States and Cuba took the first step toward restoring diplomatic relations by opening interests sections in each other's capitals, and US diplomats moved back into their seafront offices.

They were hardly welcomed back with open arms. Then-President Fidel Castro called the interests section "a nest of spies" and led frequent demonstrations in front of the building.

Castro also kept a close eye on the Americans.

"Surveillance was pretty constant," said Vicki Huddleston, who headed the interests section from 1999 to 2002. "The security officer used to say we don't even look for listening devices in the residence because the Cubans will just replace them. But it was not malicious. They just wanted to know what I was saying or other diplomats are saying."

In 2015, after President Barack Obama announced a thaw in relations, the two countries re-established full diplomatic relations and reopened embassies. Some hard-liners in Cuba -- including Fidel Castro, who had retired by then -- criticized the opening with the United States.

But many Cubans rejoiced as Americans returned to the island in large numbers for the first time in a half century. The normalizing relationship led to restored flight services and greater exchanges between the two countries.

But in November, following the US presidential election, American diplomats began to experience a series of strange incidents. As CNN first reported in August, diplomats were awoken late at night in their homes feeling unwell and hearing sounds that resembled insects or metal dragging across the floor.

They were unable to determine the source of the sound; by leaving the room or area they were in, the incidents stopped immediately, two US government officials said.

By February, the State Department had concluded their diplomats were the targets of a campaign of harassment and they needed to raise the issue with Cuban officials.

More questions than answers

The devices used in the incidents had never been found, two US officials said, but appeared to be a type of sonic weapon that emitted sound waves capable of inflicting physical harm.

But the physical symptoms that people exhibited varied greatly, preventing doctors consulted in the United States from reaching a conclusion about what caused the trauma, two US officials said.

US government technical experts were also baffled. Some affected diplomats had lines of sight to the street in their homes, while others had shrubbery and walls that blocked views of their homes. Some heard loud sounds when the incidents took place, while others heard nothing.

It does not appear either the US Embassy or the ambassador's residence were ever targeted, three senior US officials told CNN.

How much did Cuba know?

At the time, Donald Trump had just won the presidency. Raul Castro congratulated Trump on his unexpected victory in the Cuban state-run media, even though Trump had promised to take a tougher line on Cuba.

Still Cuban officials were hopeful that a modus vivendi could be reached with the new administration.

At the same time, the Cubans were racing to capitalize on the final months of goodwill from the Obama administration and sign as many agreements with the US government and American companies as possible.

It made no sense for Cuba at that moment to begin a campaign of harassment against US diplomats, US officials said.

Shortly after US diplomats complained for the first time to their Cuban counterparts, Raul Castro summoned the top US diplomat in Havana, Jeffrey DeLaurentis, to a meeting. Castro denied any Cuban involvement in the alleged attacks and promised a thorough investigation, according to US and Cuban officials.

US officials felt Castro would not have personally assured the Americans that Cuba had no part in the incidents if it had been a Cuban operation, a US official told CNN.

Since then, the United States has received cooperation from Cuba, if not many answers. The FBI was permitted to travel to the island and met with officials from Cuba's Interior Ministry, which is directing the investigation, a US official said.

Many diplomats live in Havana's upscale Siboney neighborhood, which was called Country Club before the revolution. The area's well-maintained mansions and tidy lawns are a far cry from the city's iconic decaying colonial buildings. There are surveillance cameras throughout Siboney and Cuban security guards posted in front of many diplomats' homes.

Top Cuban officials -- including Raul Castro -- have houses in the same area and are heavily guarded.

Other incidents took place in hotels in Havana where US diplomats were staying, said three senior US officials, also locations that Cuban intelligence services closely monitor.

US officials said they believe even if the Cubans didn't know about the incidents at the beginning of the investigation, they must have a clearer idea of what transpired than they are letting on.

"It is increasingly apparent the Cubans are involved in some way," a senior US official said. "The Cubans are all over our people while they are down there. If it was a few attacks, you could say that maybe it was the Russians or Iranians screwing with us, but when it happens so many times, especially in the same hotel, it is hard for us believe someone can get close enough to our people so many times. Unless these are beams from outer space."

Speaking at the UN General Assembly in New York, Rodríguez, the foreign minister, said his government "has taken into account the data contributed by the US authorities and so far has found no evidence whatsoever that could confirm the causes or the origin of the health disorders referred to by US diplomats and their relatives."

Rodríguez said Cuban officials would continue to investigate.

As Americans work to repair the US Embassy in Havana, badly damaged by Hurricane Irma, the Cuban diplomat's words this week likely provided little comfort.