Mexico City, May 9 (AP) Three sisters who worked in Mexico's government hospital system were found murdered by strangling, authorities in the northern border state of Coahuila announced on Friday, stirring new alarm in a country where attacks on healthcare workers have occurred across the nation amid the coronavirus outbreak.

Two of the sisters were nurses for the Mexican Social Security Institute and the third was a hospital administrator, but there was no immediate evidence the attack was related to their work. The state prosecutor told local media the motive might have been robbery.

State police said the bodies were found in a house in the city of Torreon. The Social Security Institute said they were killed on Thursday.

The National Union of Social Security Employees called the killings "outrageous and incomprehensible".

In other parts of Mexico, nurses have had been hit, kicked off public transport or had cleaning fluids poured on them amid fears they might spread the coronavirus. Mexican health authorities have denounced the attacks and urged medical personnel not to wear uniforms or scrubs on the street to avoid being targeted.

Meanwhile, the mayor of the border city of Ciudad Juarez, across from El Paso, Texas, announced he had tested positive for the coronavirus. Mayor Armando Cabada said he had no symptoms, but Javier Corral, the governor of Chihuahua state, said he was self-isolating as a precaution because he had contact with Cabada.

At least three of Mexico's 31 state governors have tested positive for the virus.

Ciudad Juarez has been hit hard by the coronavirus, with about two-thirds of the state's confirmed cases and 104 of its 125 deaths.

While federal authorities had predicted Mexico's caseload would peak sometime around Friday, President Andrés Manuel López Obrador said on Friday that case numbers might not plateau for another 12 days before any possible descent.

"We are at the phase of the highest contagion, we are at the peak, and according to the information we are getting, this could continue until the 20th of this month," López Obrador said. "The projection is that from that date, the number of cases would start to fall."

Mexico has almost 30,000 confirmed cases, though officials have estimated the real number may be eight times higher. The country has seen almost 3,000 deaths.

But there have been reports by several media outlets, including The Associated Press, suggesting that many coronavirus deaths go uncounted in official figures because of Mexico's extremely low-testing rate.

Assistant Health Secretary Hugo López-Gatell acknowledged that "the deaths of people with COVID cannot always be documented," but said suspected deaths would be reviewed later and perhaps added to official statistics if warranted.

He said many patients arrive at hospitals "in such serious condition that timely laboratory tests are not possible." Even though postmortem tests can be conducted, he acknowledged that is seldom done in Mexico.

"So we have people who unfortunately lost their lives, and who have had clinical conditions that suggest COVID but who are not registered because they do not have a laboratory test," López-Gatell said. (AP)

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