A U.S. congressional probe is focusing on why General Motors Co. (NYSE:GM) employees repeatedly approved substandard  ignition switches linked to at least 13 fatalities, as the automaker on Monday announced another major recall, this time related to power steering problems.

On the eve of a high-profile hearing before a House panel, GM said it is recalling more than 1.5 million additional vehicles globally, Reuters reported. That brings its total recalls so far this year to more than 6 million.

The Detroit -based automaker says it is taking an aggressive stance on safety issues, after coming under intense criticism for waiting more than a decade to recall millions of cars with potentially faulty ignition switches.

GM CEO Mary Barra, in written testimony for a House subcommittee hearing on Tuesday into the recall of 2.53 million small cars for faulty switches, says: "Sitting here today, I cannot tell you why it took years for a safety defect to be announced in that (small-car) program, but I can tell you that we will find out."

In her testimony filed in advance, Barra promises the U.S. House Energy and Commerce Committee's Subcommittee on Oversight, "When we have answers, we will be fully transparent with you, with our regulators and with our customers."

She will also offer an apology to GM's customers.

"Today’s GM will do the right thing. That begins with my sincere apologies to everyone who has been affected by this recall … especially to the families and friends of those who lost their lives or were injured. I am deeply sorry," the prepared statement reads

Barra also met with accident victims' families on Monday night in Washington, D.C., according to the company, ABC reported. 

On Monday, Democrats on the House Energy and Commerce Committee released details of some of the more than 200,000 pages of documents they have received from GM and a federal regulator, Reuters reported.

The lawmakers said they want answers as to why employees approved for production ignition switches that failed to meet company standards. These faulty parts can cause engines to stall during operation, which also disables airbags, power steering and power brakes.

Lawmakers are also exploring whether another 14 fatalities could be connected to the faulty switches, citing data from the National Highway Traffic Safety Administration regarding newer models of the recalled cars. Those deaths occurred in accidents with vehicles displaying some of the same problems as those in the earlier fatalities.

The additional 14 deaths occurred after the 13 fatalities that GM has connected to defective ignition switches.

The bad publicity around the defective ignition switches has tarnished GM's reputation even as the automaker moved to close the chapter on its 2009 bankruptcy and $49.5 billion U.S. taxpayer bailout.

With last year's launch of a redesigned version of its highly profitable large pickup trucks and the recent reintroduction of a quarterly dividend, many analysts had begun touting the company's stock.

Lawmakers on Monday highlighted inconsistencies regarding the role of a key GM engineer, Ray DeGiorgio, in a decision in 2006 to revise the design of the ignition switch.

The change was made to the portion of the switch that holds the ignition key in place as it clicks between off, accessory and on positions, called the detent plunger and spring.

In an April 2013 deposition related to a crash in Georgia involving a recalled GM car, DeGiorgio said the company "certainly did not approve a detent plunger design change" for the 2006 replacement ignition switches.

But Rep. Henry Waxman, D-Calif., and other senior Democrats on the Energy and Commerce Committee said on Monday that "GM has provided the panel with documentation verifying that a Ray DeGiorgio, lead design engineer for the Cobalt ignition switches, signed off on a Delphi ignition switch change on April 26, 2006."

GM declined to make DeGiorgio available for comment on Monday.

Barra and NHTSA acting Administrator David Friedman are set to testify on Capitol Hill Tuesday to the House subcommittee and on Wednesday to a Senate panel.