An independent but informal investigation of loan practices by officials of the National Bank of Washington yesterday disclosed a pattern of risky credit and extensions of loans to the bank's directors and those close to bank officials.

A summary of the probe, which was conducted by an independent law firm for the bank board's outside audit committee, was made public yesterday. At the same time, a copy of a lengthier report of more than 100 pages was handed over to the United States Attorney's office, which is conducting a grand jury investigation of the bank's lending practices.

In particular, the probers, attorneys for the law firm of Gibson, Dunn & Crutcher, found "no improper conduct" by leaders of the United Mine Workers of America, the labor union that holds about three-quarters of the bank's outstanding stock.

Sam Church Jr., president of the UMW, issued a statement yesterday, saying implementation of new procedures at NBW would improve bank operations. "The committee found some procedural problems with a few loan transactions," Church said. "However, the great majority of the bank's loan transactions are above question."

But the report did conclude that a $4.5 million loan to Joseph Shamy and his Laurel Harness Racing Association, Inc., former owner of the old Laurel Raceway, was made "despite the fact that many senior officers of the bank were opposed to making the loan." Shamy was convicted on seven counts of fraud last year.

Further, the August 1978 loan was presented to the bank's executive committee "without adequate disclosure of the officer's opposition and of certain adverse information known to bank officials."

Although the summary report, which was signed by seven members of the bank's audit committee, stresses that the "great majority" of the bank's loan portfolio does not suggest impropriety, in at least five cited instances, including the Laurel loan, the committee raised questions about loan procedures.

"The committee has concluded that the bank's senior management was too often too eager to please influential directors in making credit decisions," the panel's summary said. "In some instances, bank officers acted on their perception of director interest even though there is no evidence that the director had any intention to influence the credit decision."

The report was ordered by the bank's board last April in light of allegations that surfaced last spring about a series of questionable loans by NBW, the city's third largest bank, with a loan portfolio of more than $500 million. But the committee and the law firm could not compel testimony and did not take sworn depositions, and there was no cross-examination in more than 140 interviews conducted in connection with the probe.

Among other findings of the report:

"Several" loans of the total $2 million loaned to former NBW director Bruce Lyons, partner in the troubled real estate firm of Holland and Lyons, and associates were made "without adequate financial information and without careful credit analysis" by bank officiers.

"The committee has concluded that the bank failed to understand these loan transactions and did not appropriately document them," the report said. The auditors found that neither Ronald G. Nathan, who recently resigned from his post as NBW general counsel, nor any other bank director "actively intervened in the loan approval process for these loans."

A 1978 loan of $1.19 million to Marvin Goldman, owner of K-B Theaters and a director of the bank, was made "despite unanswered questions as to the debt sevice of the borrowers and was structured in such a manner that repayment at maturity was not feasible." The loan was restructured in 1979 to be repaid by September 1980.

A series of loans, which according to an NBW proxy statement totaled more than $3 million, to John W. Lyon, president of Excavation Construction Inc., and termed in the report a "valued customer of the bank," were called "unduly lenient, including decisions as to loan security and repayment schedules."

The audit committee concluded that there was "no evidence of impropriety" on the part of Lyon, but also said that bank officers were influenced by his position as director and as chairman of the bank's Salary and Nominating Committee.

Nathan, the audit commitee said, "did not have an undisclosed interest" in a gas well investment linked to former board member Bruce Lyons. But the committee, without giving detail, saaid it "concluded that in connection with several transactions" Nathan's "conduct is subject to criticism."

In releasing the report, NBW also announced that its board had adopted a series of procedural changes in bank management, including the establishment of a "board loan approval committee" to approve transactions with board members. In addition, the bank said it would strengthen board procedures, set up "standards of professional judgment and conduct" and add new precautions for director loans.

Asked if the subject of his future had come up at a bank board meeting, NBW Chairman Dale Jernberg said: "I'm in good shape." Jernberg would not elaborate on the audit committee summary.