President Donald Trump said Wednesday that the U.S. does not believe a bioweapon caused the death of a worker at a plague research lab in Siberia. "We don't think so. We're going to find out soon enough, but we don't think so," Trump told reporters in the Oval Office, CNBC and France 24 reported. He said he had a call set up with Russian President Vladimir Putin, who turned 74 on Wednesday, CNBC reported. Earlier, the Kremlin said no call had been set yet but one could be arranged quickly, the AP and Reuters reported.

The United States has also sent Russia a formal complaint over the death of a worker at a plague research lab in Siberia. The note, called a démarche, asks Moscow for more facts about what happened, ABC News and CNN reported. The Wall Street Journal first reported it.

"We have officially requested additional information from the Russian government," a State Department spokesperson said. "We are also in official communication with other countries around the world to share information."

The worker, Darya Shipilova, 28, was a technician at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East. She died Oct. 2 of pneumonia, and officials have not named the cause. U.S. officials told the Journal that Russia has shared little, and that this goes against its duties under world health rules.

On Tuesday, Trump had said the call would come "very soon," NBC News reported. The Centers for Disease Control and Prevention says it is watching what it calls a suspected case of pneumonic plague.

Russia's health agency, Rospotrebnadzor, says tests found no germs from her lab work in her samples. Russia has told the World Health Organization there have been no recent plague cases in the Irkutsk region. The WHO has asked Russia what caused her illness and whether a second worker is sick. About 200 people were placed under watch after her death. On Wednesday, the agency said none of them had plague and it began easing safety steps, CBS News reported. The WHO says the risk to the public appears low.

The hospital near Irkutsk where she was treated was put under quarantine, Fox News reported, citing The Associated Press.

The backstory: Plague is caused by a germ called Yersinia pestis, usually carried by rodents and their fleas, and it turns up from time to time in Siberia. The lung form, pneumonic plague, can spread between people and can kill quickly without early care, though antibiotics work when it is caught early. The Irkutsk institute was founded in 1934 to fight plague, and its staff also study other dangerous diseases, such as cholera and anthrax, CNN reported.

What we know: Trump says the U.S. does not think a bioweapon was involved, and Russia says it found no plague.
What we don't: What killed the worker, whether a second worker is ill, and what Trump and Putin will say after they talk.