For more than a month, the Centers for Disease Control and Prevention has been offering to send a team of experts to China to observe its coronavirus outbreak and help if it can.
Normally, teams from the agency's Epidemic Intelligence Service can be in the air within 24 hours.
But no invitation has come - and no one can publicly explain why.
The World Health Organisation (WHO), which made a similar offer about two weeks ago, appears to be facing the same cold shoulder, though a spokeswoman said it is just "sorting out arrangements".
Current and former public health officials and diplomats, speaking anonymously for fear of upsetting diplomatic relations, said they believe the reluctance comes from China's top leaders, who do not want the world to think they need outside help.
Bumps in the road
In 2003, China was badly stung by criticism of its response to SARS, another coronavirus epidemic; it has also been embroiled in a trade war with the United States for more than a year.
Some experts also say that outsiders could discover aspects of the outbreak that are embarrassing to China: for example, the country has not revealed how many of its doctors and nurses have died fighting the disease.
But China does need help, experts argue.
In private phone calls and texts, some Chinese colleagues have indicated that they are overwhelmed and would welcome not just extra hands, but specialised expertise in a couple of fields.
Also, CDC officials have said that they hope to learn more about the new coronavirus from their Chinese counterparts to improve the American response if the virus starts to spread widely here.
On Friday, Alex Azar, secretary of health and human services, said at a news briefing that he had recently reiterated the offer of a team to his Chinese counterpart, Dr Ma Xiaowei.
Asked what the holdup was, he answered: "It's up to the Chinese. We continue to expect fully that President Xi will accept our offer. We're ready and willing and able to go."
On Jan. 29, Tedros Adhanom Ghebreyesus, the WHO's director general, returned from a brief trip to China full of praise for President Xi Jinping and the country's extraordinarily aggressive response, which has nearly walled off Hubei province from the rest of the country and the world.
China had "agreed to a mission of international experts" to better understand disease transmission and clinical severity, Dr Michael Ryan, the WHO's emergency response chief, said at the time.
Asked if that team would include American experts, Tedros replied that "best would be a bilateral arrangement".
On Thursday, a WHO spokeswoman said that there was no delay in the organisation's own mission to China.
"Our understanding is that the mission is on," Marcia Poole, the spokeswoman, said. But she could not say when the team would leave or who would be on it.
"It's a matter of sorting out the arrangements," she added, noting that Ryan had said a team would "need representation from North and South, East and West, with relevant areas of expertise."
The United States has offered Tedros 13 specialists who are ready to go, Azar said.
The two fields in which China appears to need outside help, experts said, are molecular virology and epidemiology.
The first involves sequencing the virus' genome and manipulating it to refine diagnostic tests, treatments and vaccine candidates.
The second involves figuring out basic questions like who gets infected and who does not, how long the incubation period is, why some victims die, how many other people each victim infects and how commonly hospital outbreaks are occurring.
"This isn't rocket science, it's basic stuff - but it's been five weeks and we still don't know the answers," one expert said.
It would be very useful, for example, to have a blood test for antibodies. That would make it possible to see how many infected people had recovered, which would make it clearer as to how lethal the virus is - and how widespread.
A major epidemiological failure by China is that the Wuhan authorities appear to have closed and disinfected the seafood market that was the outbreak's early focus without swabbing individual animals and their cages and without drawing blood from everyone working there. That would have provided a wealth of information about which animal might have been the source of the coronavirus and which people had become infected but survived.
Asked what had happened to the animals - whether they had been burned or buried, for instance, one expert said: "No one can tell me that. I don't think they know."
China has greatly improved its ability to fight disease since it was embarrassed by SARS, and its scientists now frequently publish in major medical journals.
Many of them trained at the American CDC and have friends there.
"We have a decadelong relationship," Azar said Friday. "It's not an accident that it's called the Chinese CDC."
Experts raised a related concern: China's scientists are given large rewards for publishing in prestigious journals. That creates an incentive to hold back samples and data until publication. Although the American CDC has in the past sometimes had fraught relations with other countries because it used their samples and published analyses of them without giving credit they felt was due, its first priority was still to issue epidemic warnings if they were needed and then to publish later.
"In an epidemic, you don't want information held back," one expert said. "You want transparency."