Two more soldiers overseas have come down with serious pneumonia (search), bringing the unexplained cases to 17, the Army said Monday.

Officials are investigating the cause of some 100 cases counted since March, focusing on a number of them so serious the patients had to be put on ventilators and flown to Europe (search).

The number of serious cases was 15 last week and now has risen to 17, said a statement Monday from the Army surgeon general's office.

The statement said officials have found "no infectious agent common to all of the cases," and no evidence the patients were exposed to biological or chemical weapons, Severe Acute Respiratory Syndrome (search) (SARS), or environmental toxins.

Officials said last week that cases were among troops serving in the conflicts in Afghanistan and Iraq, with 10 of the then-15 cases from Iraq and the others from Uzbekistan, Qatar and elsewhere. Monday's statement didn't say where the new cases happened, and no one was available to comment.

A two-person investigative team has gone to Landstuhl Regional Medical Center in Germany, where most of the sick soldiers were treated after evacuation. A six-person team was went to Iraq, including infectious disease experts, laboratory workers and people to take samples of soil, water and air as well as medical samples from patients.

The two teams were to review patient records and laboratory results and interview health care workers and patients, if possible. The U.S. Centers for Disease Control and Prevention also is reviewing the cases.

Armywide, between 400 and 500 soldiers get pneumonia each year. It is the severity of these new cases that has caused special concern.