National Highway Traffic Safety Administration officials say they are expanding their investigation into possible steering-column defects in Mazda passenger cars.

The investigation -- a preliminary probe, or "engineering analysis" in NHTSA parlance -- originally involved Mazda cars manufactured for the 1976 through 1981 model years. Now Mazdas through the 1984 model year will be included.

According to the allegations, the steering columns can lock while the cars are moving, causing the driver to lose control. Mazda repeatedly has denied that the defect exists.

A NHTSA spokesman said Monday that the agency has received reports of 12 steering-column failures in Mazda cars, including two accidents involving serious injury to the passengers.

The latest was a crash involving a 1984-model Mazda 626 LX in Rock Creek Park in Washington on Jan. 27, 1984. That accident, in which the driver suffered brain damage, indirectly led NHTSA to widen its Mazda probe.

The Washington-based Center for Auto Safety, a consumer advocacy group, urged NHTSA to examine the steering column in the car for possible signs of a locked-up steering column. Washington lawyer Milton Heller, who is representing the victim in the crash, has charged that his client lost control of the car after the steering wheel locked for no apparent reason.

In congressional testimony on Aug. 8, NHTSA Administrator Diane K. Steed and Associate Administrator George Parker contended that their agency's investigation into alleged Mazda defects had been hindered because NHTSA had had trouble acquiring the steering columns in cars that had been involved in accidents.

NHTSA turned down the consumer group's request that the agency examine the steering column in the car in the Rock Creek Park accident on grounds that Heller is seeking monetary damages on behalf of his client.

"It is our policy not to get involved in anything where direct litigation is involved," a NHTSA spokesman said. "We don't want to do anything to screw up the litigation."

However, as a result of the consumer group's request, NHTSA agreed to include the newer cars in its investigation, the agency spokesman said.

Meanwhile, in Greeneville, Tenn., the woman who launched efforts for a federal inquiry into the safety of the Mazda steering columns has lost again in court. A state appeals court in Knoxville recently turned down Phyllis Painter's request for a rehearing of a local court case in which she charged that her 1978 Mazda GLC had crashed when its steering column locked up.

Painter, who was seriously injured in the accident, had argued that the lower court's decision was based on incomplete information. Undaunted by her reversals, she filed a federal suit on Monday against Yushin Co. Ltd. of Hiroshima, Japan, a Mazda supplier that made the lock that she says caused her accident.

MEANWHILE, IN THE GM CASE . . . Another big pea that does not seem to want to go away is the General Motors Corp. X-car case, in which the 70th day of testimony concluded yesterday. The nonjury trial here before U.S. District Court Judge Thomas P. Jackson began last March 13 but has been interrupted for long periods.

The government has accused GM of marketing 1980-model X-cars with rear brakes that it knew were defective and could lock, causing the driver to lose control. NHTSA, with help from the Justice Department, is seeking $4 million in fines and the "immediate" recall of all 1.1 million X cars that it suspects have the problem.

GM has denied all charges in the case, and company officials now seem confident that they have a good chance of winning it.

Even if the trial ends later this month, as some predict, the load of testimony and data could mean another two months or more before Jackson can render a decision, some sources say.