Health authorities in Hong Kong are investigating the outbreak of a mysterious respiratory illness that has infected 28 children in a hospital.

Tests conducted late last week showed the children were not infected with SARS or influenza. “We will see if they need to be further isolated, and to see if we need staff reinforcements,” Health Minister York Chow told reporters on Monday.

The children are all long-term residents at two wards in the Caritas Medical Center, a hospital for mentally handicapped children.

Four more children developed fever and symptoms of respiratory infection on Monday.

An 11-year-old girl, who had chronic medical problems, died of a chest infection in the same hospital on November 9, but authorities have yet to confirm if the girl died of the same disease now afflicting the other children.

“Tests on the children have turned up negative for SARS, influenza A and B, and other respiratory related diseases. We are still checking,” a health department spokeswoman said.

Some local newspapers have sharply criticized the government for its handling of the case, which evoked memories of health disasters that have dogged Hong Kong in recent years, including SARS and the deadly H5N1 bird flu.

Hong Kong was badly hit by severe acute respiratory syndrome last year. The new disease, which originated in southern China, infected nearly 1,800 people in Hong Kong, killing 299 of them.

Inquiries into the outbreak highlighted deficiencies in the healthcare system and a poor chain of command.