The assailant also stabbed and injured a Chinese tour guide with the Americans. He then committed suicide by leaping from a 130-foot-high balcony of the ancient landmark the Americans were visiting, the 13th-century Drum Tower, the official Xinhua News Agency reported.
Xinhua said the injured tour guide was also a woman, it said. The volleyball teams is "deeply saddened and shocked," said Darryl Seibel, a USOC spokesman.
He said the two victims "were not wearing apparel or anything that would have specifically identified them as being members of our delegation" or as Americans.
The U.S. Embassy said it believed the attack was an isolated act and not directed at Americans or foreigners, given that the Chinese tour guide was also hurt.
"We don't believe this was targeted at American citizens, and we don't believe this has anything to do with the Olympics," embassy spokeswoman Susan Stevenson said.
The killing was a rare instance of violent crime against foreigners in tightly controlled China, which has ramped up security measures even more for the Olympics.
The attack shortly after noon at the busy tourist site darkened the mood at the games the day after a spectacular opening ceremony had set an ebullient tone after years of nervous buildup.
Beijing's communist leaders are hypersensitive to anything that could take the shine off the games, insisting issues such as China's human rights record, harsh rule in Tibet and ties with Sudan should not be raised at the sports event.
China's Foreign Ministry had no immediate comment on the attack. Xinhua identified the attacker as Tang Yongming, 47, from the eastern city of Hangzhou. It said Tang attacked the two Americans and their Chinese tour guide on the second level of the ancient tower, then leapt to his death immediately afterward.
Seibel said it was "too early to say" whether security would be stepped up for the U.S. team. But some athletes were already thinking about it.
Jennie Finch, a member of the U.S. softball team, said her heart skipped a beat when she heard about the attack, but she was undaunted.
"I'm here with my husband and son, so it's not easy but we're living our dreams and we're not going to live in fear," she said. "We're going to go out there every day and enjoy every day and celebrate it."
U.S. Ambassador Clark T. Randt visited the victims at a hospital in Beijing to convey the condolences of President Bush, who is in Beijing for the first days of the games.
"Our thoughts and prayers are with the victims and their families," Bush told reporters. "And the United States government has offered to provide any assistance the family needs."
"It is impossible to describe the depth of our sadness and shock in this tragic hour," said USOC Chairman Peter Ueberroth. "Our delegation comes to the games as a family, and when one member of our family suffers a loss, we all grieve with them."
At the scene, police blocked off streets leading to the Drum Tower, which is just five miles from the main Olympics venue in China's capital. They cordoned off the area with yellow police tape while officers collected samples from the tower and the street below.
Some details of the attack, including the motive and weapon used, were not immediately clear.
Attacks on foreigners in China are extremely rare. A Canadian model was murdered last month in Shanghai - police said she had stumbled onto a burglary.
In March, a screaming, bomb-strapped hostage-taker who commandeered a bus with 10 Australians aboard in the popular tourist city of Xi'an was shot to death by a police sniper. Shanghai and Beijing are still safer than most foreign cities of their size. Punishments for crimes against foreigners are heavier than for crimes against Chinese, and citizens are not allowed to own guns.
Even so, the U.S. government now warns Americans against muggings, beatings and even carjackings, especially in the nightlife and shopping districts of large cities.
The Drum Tower is one of few ancient structures still in fast-developing Beijing. Drummers pounded their massive instruments on the hour to let people in the imperial city know the time.
