Potentially explosive airbags manufactured by Takata Corp. of Japan and used by more than a half-dozen automakers are forcing the National Highway Traffic Safety Administration to intensify an investigation of nearly 1.1 million vehicles.

NHTSA said that its Office of Defects Investigation received a complaint last August of a driver’s bag inflator rupture in a 2005 Honda Civic. In March, ODI officials indicated they received another complaint alleging a passenger’s bag rupture on a 2003 Toyota Corolla. Then in April, ODI received a third complaint alleging a driver’s bag rupture in a 2005 Mazda 6.

“There were three alleged injuries from these three incidents and all appeared to be minor in nature,” NHTSA officials said.

ODI noted that it then discussed these incidents with Takata, the supplier of airbags involved and with the affected vehicle manufacturers.

In the course of its review, NHTSA said Takata identified two other incidents, one involving a passenger bag rupture on a 2004 Nissan Sentra and another a driver’s bag rupture on a 2006 Dodge Charger. Toyota also provided another passenger’s bag rupture on a 2002 Toyota Corolla, according to NHTSA.

“Of note, all six incidents occurred in a high absolute humidity climate (Florida and Puerto Rico),” officials said. “By way of background, several manufacturers in recent years have conducted safety recalls of vehicles for rupturing airbags.”

In 2008 through 2011, NHTSA recapped,Honda conducted a series of recalls concerning driver’s bag inflator ruptures on various 2001 through 2004 vehicles. In 2013, Honda, along with Toyota, BMW, Nissan and Mazda, initiated safety recalls to address passenger bag ruptures in certain 2001 through 2004 models.

“None of these recalls were regional in nature or attributable to atmospheric conditions in field use,” federal officials said.

In response to the investigations, Takata government affairs specialist Mike Rains used a letter to the agency to recap a meeting the company had with NHSTA this past May.

“At that meeting, Takata pointed out that all six of the potentially relevant rupture incidents had occurred in either Florida or Puerto Rico. Takata also described the research being conducted to better understand the causes of those incidents,” Rains said.

“Among other things, Takata explained that Florida and Puerto Rico have exceptionally high levels of absolute humidity, and that exposure to that level of humidity, in conjunction with potential processing issues during certain manufacturing time periods that may influence aging stability, are the focus of Takata’s current research and investigation efforts,” he continued. “At that meeting, NHTSA indicated that it was willing to work with Takata as the company’s investigation proceeds over the next several months.”

Rains also acknowledged the airbags associated with these investigations were utilized by seven OEMs, including BMW, Chrysler, Ford, Honda, Mazda, Nissan and Toyota.

“Takata wants to assure you that it will work with each manufacturer that agrees to conduct a field action to develop a schedule for that action, based on parts availability,” Rains went on to say in the letter to NHTSA. “In addition, Takata plans to continue its wide-ranging investigative efforts to better understand the causes of these inflator incidents, and it will keep ODI informed of the progress of those efforts.”