A nationwide probe of suspected defense contractor fraud and bribery is focusing on how companies gained inside tips from the Pentagon to win contracts, law enforcement sources said Wednesday.

The FBI and Naval Investigative Service inquiry is expected to turn to Congress soon to learn if lawmakers or their aides were involved in leaking sensitive contract information, sources said.The investigation has to do with contractors getting advance information on bid packages and basic Pentagon budget data helpful in preparing contract bids, an administration law enforcement source said.

Investigators said the use of bribes to get contract information, including specifications, also is a target of the probe.

Congress was swept by rumors Wednesday that search warrants were to be issued for records in the armed services committees and the defense appropriations subcommittees. Staff members for the committees said they had heard the reports, but had not seen any warrants yet.

FBI spokesman Greg Jones said Wednesday no warrants had been issued for congressional rec-ords but declined to say whether such warrants would be issued.

"A lot of the information needed by contractors to prepare bids could come out of those committees," said one defense procurement expert.

President Reagan said Wednesday that while he was surprised to learn of the investigation, he fully backs attempts to root out defense contractor corruption. White House spokesman Marlin Fitzwater said Reagan was briefed by Defense Secretary Frank Carlucci Tuesday after the FBI disclosed the investigation.

Investigators tapped Pentagon offices with electronic eavesdropping devices for over two years as part of the investigation in military purchasing, White House officials said.

Many of the nearly 250 FBI and 75 NIS agents involved in the two-year probe have now begun reviewing the documents obtained by court-ordered warrants on Tuesday and Wednesday.

FBI spokesman Jones said two search warrants were issued Wednesday, authorizing a search of records at the office of James Roberts of Fairfax, Va., a consultant to defense contractors, and James Rapinac of Bridgeport, Conn., also a consultant.

On Tuesday investigators searched records of government officials, contractors and consultants in 12 states on authority of 31 court-ordered warrants.

The Pentagon office of Victor Cohen, the civilian official responsible for buying tactical battle command, control, communications and computer systems for the Air Force, also was searched.