It's not clear how the nurse contracted the virus, though the second case among health workers pointed to lapses beyond how one individual may have donned and removed protective garb.
Authorities declined to say what type of care the nurse provided to Thomas Eric Duncan, who was diagnosed with Ebola after coming to the U.S. from Liberia. He died Oct. 8.
Even though the nurse did not report having a fever until Tuesday, the day after she returned home, she should not have boarded a commercial airliner after learning that another nurse, Nina Pham, had been diagnosed with Ebola, government officials said Wednesday.
CDC Director Tom Frieden said no one else involved in Duncan's care will be allowed to travel "other than in a controlled environment."
Infected Ebola patients are not considered contagious until they have symptoms. Frieden said it was unlikely that other passengers or airline crew members were at risk because the nurse did not have any vomiting or bleeding.
Even so, the CDC is alerting the 132 passengers aboard the Frontier Airlines Flight 1143 from Cleveland to Dallas-Fort Worth on Monday "because of the proximity in time between the evening flight and first report of illness the following morning." Officials are asking them to call the health agency so they can be monitored. The woman flew from Dallas to Cleveland on Oct. 10.
Also Wednesday, Kent State University said the second nurse was related to three university employees and that they were asking the workers to stay off campus for 21 days "out of an abundance of caution."
The federal Centers for Disease Control and Prevention said the additional infection is "a serious concern."
"What happened there (in Dallas), regardless of the reason, is not acceptable. It shouldn't have happened," Anthony Fauci, director of the Institute of Allergy and Infectious Diseases of NIH, said Wednesday on MSNBC.
The nurse reported a fever Tuesday and was in isolation within 90 minutes, Dallas County Judge Clay Jenkins said.
"We are looking at every element of our personal protection equipment and infection control in the hospital," said Dr. Daniel Varga, chief clinical officer for Texas Health Resources, which operates Texas Health Presbyterian Hospital Dallas.
In Washington, the White House said Obama's trip to New Jersey and Connecticut would be postponed to a later date. The president was to meet with top officials who are coordinating the government's response to Ebola.
His decision to nix the trip - just a few hours before Air Force One was scheduled to depart - reflected the urgency of the situation amid escalating concerns about the disease.
The second nurse will be transferred to a special bio-containment unit at Emory University Hospital in Atlanta, where other Ebola patients have been successfully treated, Frieden said. Pham will be monitored in Dallas to determine the best place for her care, Frieden said.
The CDC has acknowledged that the government was not aggressive enough in managing Ebola and containing the virus as it spread from an infected patient to a nurse at a Dallas hospital.
"We could've sent a more robust hospital infection control team and been more hands-on with the hospital from day one about exactly how this should be managed," he said Tuesday.
The second case may help health officials determine where the infection-control breach is occurring and make practices safer for health workers everywhere. For example, if both health workers were involved in drawing Duncan's blood, placing an intravenous line or suctioning mucus when Duncan was on a breathing machine, that would be recognized as a particularly high-risk activity. It might also reveal which body fluids pose the greatest risk.
Emergency responders in hazardous-materials suits began decontamination work before dawn Wednesday at the Dallas apartment complex where the second nurse lives. Officials said she lives alone with no pets.
Notices handed out to neighbors advise that "a health care worker who lives in your area has tested positive for Ebola."
Ryan Fus, 24, who lives in the same building as the blocked-off apartment, said police knocked on his door before 6 a.m. to notify him and make sure he was all right.
"It's a little scary. It's a little shocking that it's right near me," he said.
Dallas city spokeswoman Sana Syed said a hazardous-materials crew has finished cleaning common areas of the complex and that the state was sending a crew to clean the actual apartment.
Frieden outlined new steps this week designed to stop the spread of the disease, including the creation of an Ebola response team, increased training for health care workers nationwide and changes at the Texas hospital to minimize the risk of more infections.
The new case lends support to nurses' claims this week that they have inadequate training and in some cases, protective gear, to take care of Ebola patients.
"They're not prepared" for what they are being asked to do, said RoseAnn DeMoro, executive director of National Nurses United, a union with 185,000 members.
Based on statements from nurses it did not identify, the union described how Duncan was left in an open area of the emergency room for hours. It said staff treated Duncan for days without the correct protective gear, that hazardous waste was allowed to pile up to the ceiling and safety protocols constantly changed.
Officials say at least 76 people at the hospital might have been exposed, and all are being monitored for fever and other symptoms.
Pham, 26, released a statement Tuesday through the hospital saying she was "doing well." She was listed in good condition.