At least 13 congressmen actively assisted South Korean businessman Tongsun Park's extensive lobbying and business interests according to "concrete, tangible" evidence developed in the Justice Department investigation of Korean influence-buying in Washington, informed sources say.

The documentation is the first evidence of a wide-spread pattern of congressional assistance to Park. It complements previously assmebled evidence that Park spent between $500,000 and $1 million a year in cash, campaign contributions, gifts and lavish entertaining to influence congressional action on legislation affecting South Korea.

The assistance by congressmen allegedly involves intervening with government agencies and departments, lobbying their fellow congressmen, approaching officials of other governments, promoting corporate business interests of U.S. and Korean corporations with which Park was involved, and providing Park with information normally restricted to government channels.

Park allegedly received assistance in purchasing rice and other commodities for South Korea, counteracting criticism of South Korean president Park Chung Hee's actions, maintaining and increasing U.S. defense levels in South Korea, avoiding tariffs on South Korean goods, securing contracts funded by U.S. aid, and avoiding the effect of the regulations and restrictions imposed on Park's own business activities by the Agriculture and Commerce Departments.

To prove any illegality, prosecutors must prove that legislators specifically received something of value as a reward for their assistance. However, much of the evidence accumulated by prosecutors may not be admissible in court because of the constitutional prohibition on questioning legislators about their legislative activities anywhere but in Congress.

Nevertheless, prosecutors are encouraged that the newly acquired evidence of a pattern of congressional assistance to Park will allow them to find out what happened, regardless of whether it leads to criminal indictments.

In the past few months, the Justice Department began focusing on four to six congressmen who were intimately involved with Park and other Korean agents. Although no firm list has been drawn up by prosecutors, those generally believed to be the subjects of the greatest scrutiny are former Reps. Edwin W. Edwards, (D-La.), now governor of Louisiana; Cornelius E. Gallagher (D-N.J.); Richard T. Hanna (D-Calif.); William E. Minshall (R-Ohio); Otto E. Passman (D-La.), and current Rep. Robert L. Leggett (D-Calif.).

In the past few weeks thousands of additional pages of evidence have been accumulated, much of its resulting from subpoenas for materials in Park's personal residence. The Internal Revenue Service seized the residence in February in the execution of a tax lien against Park. Justice also has received an extensive number of documents from congressmen, present and former congressional aides, associates, of Park, and several witnesses identified by the FBI.

The new material adds several more names to the list of those who have allegedly aided Park. They are former Sens. Allen J. Ellender (D-La.), now deceased; Jack Miller (R-Iowa), now a federal judge in the U.S. Court of Customs and Patent Appeals; Joseph M. Montoya (D-N.M.), who was defeated for re-election last year; former Reps. Nick Galfianakis (D-N.C.); Seymour Halpern (D-N.Y.); David Pryor (D-Ark.), now governor of Arkansas, and current Rep. John J. McFall (D-Calif.), majority whip until earlier this year.

SInce early 1975, the Justice Department has been investigating allegations that Park was acting as a foreign agent on behalf of the South Korean regime of Park Chung Hee and, with other agents, hads spent millions of dollars during the 1970s to influence U.S. congressmen and other officials.

Last December, then President-elect Jummy Carter was told officially that 18 congressmen were under investigation in the probe, well down from earlier estimates in the press that up to 90 congressmen were under investigation.

Recent press reports that the investigation of congressmen will likely lead to only a couple of income tax prosecutions, leaving further investigation to congressional committees, are "naively premature," according to Justice Department sources.

However, interviews with sources in the Justice Department and the intelligence field and a review by The Washington Post of a significant portion of the thousands of pages of new evidence - made available by mongovernmental sources - indicate that several factors have been hampesring and delaying the investigation. They are:

Despite extraordinary cooperation from the CIA in providing Justice Department access to files, some apparently strategic information has apparently not been included in those files.

It is thought that knowledgeable officials in State, Defense and the CIA have not offered critically important information to investigators, apparently because they fear that their own failure to expose the Korean plot may damage their careers.

Park's complicated relationships with his congressional allies makes it difficult for investigators to distinguish between favors performed for Park and favors done for constituents back home. Park was unusually clever, according to sources, at matching his own interests - such as rice - with those of an interest group in each congressman's district.

Prosecutors have been reluctant to request or subpoena files and financial records from present and past congressmen involved with Park. As a result many files have been purged of embarrassing information before being turned over to the Justice Department, according to congressional sources.

Investigators have been cautious in their use of subpoena to secure information from Park and his business associates. As a result, many relevant documents were either destroyed or taken to London when Park went there, according to his associates.

Many key witnesses, including associates of Park, congressional aides, and other officials have not been interviewed due to the limitations of staff and the difficulty of managing the logistics of such a large and complicated investigation. As a result, several hundred potential witnesses remain to be interviewed.

The tremendous volume of financial records and other documents obtained will take months to finish cataloguing and analyzing. Moreover, the complicated interrelationships between dozens of congressmen, dozens of Korean agents and diplomats, and the numerous SOuth Korean military and economic priorities has created a maze of tightly interwoven transactions whose meanings may take months to decipher.

Prosecutors and State department officials have been reluctant to use diplomatic channels to encourage cooperation from the Korean government! As a result, several witnesses have left the U.S. to return to Seoul where they are beyond subpoena power. One potential witness, allegedly with direct eyewitness knowledge of cash payoffs to congressmen, has not been allowed to leave South Korea.

Many man-months of valuable investigative time is being lost checking leads, which, although they come from reliable witnesses, may be unverifiable. The leads include allegations that many other congressmen and at least three White House aides received funds from Korean agents.

The most fundamental criticism of the investigation from within the Justice Department has been the failure to single out one of two particularly vulnerable congressmen for prosecution. This strategy might lead to increased cooperation from those convicted first and might encourage later defendants to plea bargaining and provide information.

Other Justice Department officials point out that the present strategy - to develop the broadest possible overview of Park's activities before selecting final targets for prosecution - has been necessary to avoid the appearance of not going after prominent officials potentially involved.

"The lesson of Watergate is that public criticism is less likely to follow a slow, methodical investigation than one in which you go after a few defendants, hoping to (use) them against others and failing," one official said. "It also happens to be the soundest way to investigate."

"The reason this damn thing takes so long is that the (alleged) conspiracy is between 10 and 100 times more complicated than Watergate, it quite literally involves hundreds and hundreds of people," another knowledgeable source said.

Former congressmen Miller, Galifianakis, Halpern, and Pryor all told The Post in telephone in terviews that they have no recollection of writing any letters in support of Park's activities or otherwise assisting him, but acknowledge they may have, Pryor said he may have assisted Park indirectly by assisting Arkansas rice growers.

Miller said Park suggested at some point that he write a letter to Park Chung Hee congratulating him on the success of his regime and may have suggested that Miller mention Park's name. Miller does not recall sending any such letter, but said he may have.

McFall has previously denied helping Park except in facilitating rice sales from California rice fields through companies affiliated with Park.

Hanna has acknowledged working with Park in several business ventures and receiving substantial income from the ventures.

Minshall has acknowledged doing favor for Park but has denied taking any money from him.

Other congressmen could not be reached for comment.

Prosecutors are reportedly concerned about the effect of the "speech of debate" clause of the Constitution, which provides that "for any speech or debate in either House, they (senators and representatives) shall not be questioned in any other place."

The protection provided by the clause is generally agreed to extend to prohibit criminal investigation into any legislative activity such as voting, committee reports, congressional investigations. Similarly, legislative acts may actually not be even cited against them as evidence of a corrupt act.

Acts which are political rather than legislative in nature, such as assisting a constituent in securing a government contract, are generally not protected from inquiry.

However, court have held several times that legislators may be prosecuted for accepting money either in return for promising to act in a certain way, (accepting an illegal gratuity).

Since the prosecution must avoid citing legislative duties in such cases as part of the evidence, they need only prove that the money was accepted, in the case of a bribe, in return for the promise to act or, in the case of an illegal gratuity, as a reward.

Prosecutors do not need to prove that the actual legislative act promised or rewarded actually took place.

Press reports that Park's departure from the U.S. for London last fall has hampered the investigation are incorrect according to Justice Department sources. Park, they note, could be extradited from London, if and when he is indicted by a grand jury. However he is not expected to cooperate even if returned to the U.S.

Investigators have had difficulty corroborating the testimony of the former second ranking KCIA agent in the U.S., Kim Sang Keun, who last November began cooperating with the investigation in return for assistance in remaining in the U.S. However, Kim's help has been invaluable in providing leads to the officials to whom Park and others allegedly gave funds. Park and others allegedly gave funds. Those parts of Kim's testimony that can be verified have checked out, sources say.

Prosecutors have had some success in tracing the complicated series of financial transaction apparently intended to disguise the source of funds. One congressman reportedly moved relatively small amounts of funds through a series of transactions - checks, arline tickets, etc. - finally ending up with the money in cash.

Since the transaction left a paper record, the Justice Department has been able to trace the transaction back to a foreign principal.

In another instance, a congressional aide asked by a congressman to return funds given him by a Korean diplomat was inadvertently told by the Korean Embassy the names of other congressmen visited by the diplomat that day. The aide has reportedly reconstructed the list for the Justice Department.

Several of the many witnesses appearing before the grand jury have been granted immunity from prosecution in return for their testimony. They are reportedly providing varying degrees of cooperation.

Evidence collected by the Justice Department is also subject to subpoena by the two House committees investigating aspects of the South Korean influence-buying here; since House committees are not bound by the constitutional structure against questioning legislative activity, much of the evidence may become public in the next year through the committee hearings.

Given the complexity of the case, some aspects of the Justice Department investigation are likely to go on as long as another year, sources said.