Federal investigators have transferred the focus of their probe into the DC10 from the jumbo jet that crashed in Chicago May 25 to one that was found with an unusual crack in the engine mounting system as the result of the inspection, they said yesterday.

Furthermore, both the National Transportation Safety Board and the Federal Aviation Administration have intensified their studies of the DC10's hydraulic system, including the operation of the wing slats.

Slats are devices on the wing that extend during takeoff and landing to increase the plane's lifting ability. The slats on the left wing are thought to have failed on the plane that crashed.

The large, unusual crack was discovered on a United Airlines DC10 inspected at O'Hara International Airport in Chicago after the crash of American Airlines Flight 191. A 10 1/2-inch crack was discovered on the aft pylon bulkhead of the American plane, and inspections of the pylon area were ordered on all DC10s.

A large crack in the spar web area - supporting material on top of the pylon - was found on the United Plane. A study of the plane's history showed that it had flown for more than an hour after the wing engine on the cracked pylon stopped suddenly. That United plane was the only DC10 discovered during the inspections to have that problem.

"We don't really understand what caused that crack," a top FAA official said yesterday. The safety board has the same question.

Other cracks discovered on inspected DC10s have been related to the pylon bulkheads, or attachment bearings and bolts that had already been identified by McDonnell Douglas, the manufacturer, as items to be replaced.

The central question of when the grounded U.S DC10 fleet will be returned to service had no clear answer yesterday. McDonnell Douglas turned over the FAA documents and technical details that had been requested as part of the FAA's review of its own initial certification of the DC10 in 1971.

FAA chief Langhorne M. Bond flew to Los Angeles yesterday to participate in that study, but said through a spokesman that no decisions will be reached or announced in Los Angeles. FAA teams have completed a study of airline maintenance practices concerning the DC10, but their report is not complete, Bond's spokesman said.

Maintenance procedures have been blamed for the cracks discovered in the aft pylon bulkhead. Along with that discovery is a concern, expressed privately by federal officials, that the bulkhead area may be too sensitive to damage in normal maintenance work.

The study of the design material cannot be completed before the end of this week, Bond's spokesman said, and decisions about how to proceedbeyond that will come next week at the earliest.