Federal regulators are stepping up the timetable for Takata to fix defective airbags in millions of U.S. cars, with priority given to the riskiest models.

The more aggressive recall schedule was announced today by the Department of Transportation. Defective parts from Takata airbags used in cars from 19 automakers have been responsible for 11 deaths and approximately 180 injuries in the United States.

“The Department of Transportation is maintaining its aggressive oversight of the efforts to recall Takata airbags as quickly as possible,” said U.S. Transportation Secretary Anthony Foxx in a released statement. “The amended order will speed up the availability of replacement airbags, and continues to prioritize the highest risk vehicles to protect the traveling public.”

NHTSA says that this is the largest, and most complex, safety recall in U.S. history.

As of December 2, a total of 12,487,086 defective Takata driver and passenger airbags have been repaired. The total number of vehicles involved is expected to reach 42 million, with between 64 million and 69 million airbags being fixed.

Some of those airbags are “like for like” recalls, meaning a car may have old airbag inflators replaced with newer, lower risk ones that will ultimately need to be replaced by redesigned units.

Even with an expedited timeline, NHTSA says the final recalls, based on this remedy order won’t be initiated until September 2020—a year later than the original projection. These last replacements will be for the “like for like” airbags considered the lowest risk.

In describing the challenges, senior NHTSA officials explained, “This is not like Lego pieces, where you can snap out a yellow one, and replace with a red one.” Given the vast scale, it has been necessary to prioritize groups for repair in order to manage production.

Senior NHTSA officials on a conference call explained that the completion rate could be higher at this point if consumers were more responsive to recall notices. NHTSA is providing guidance to help automakers communicate to consumers when a recall can be performed.

Recall completion rates in general are considered to be too low. NHTSA recommends all car owners visit safercar.gov twice a year to proactively check for recalls using an online VIN lookup tool.