(Adds details of acting CEO's letter to investors)

By Heather Somerville and Rishika Sadam

May 16 (Reuters) - The U.S. Department of Justice has opened an investigation of Lending Club Corp, the online lender whose chief executive, Renaud Laplanche, was forced out last week after an internal probe found the company had falsified documentation when selling a package of loans.

The company also said in a quarterly filing with the Securities and Exchange Commission on Monday that a number of its large investors had halted investments in Lending Club loans in the wake of Laplanche's resignation, which could have a material effect on the company's performance going forward.

Lending Club said it did not know what impact the slowdown in investment will have going forward or whether the investors will return.

The San Francisco-based company is the largest of so-called marketplace lenders, which sell their consumer and small-business loans on to investors.

Lending Club said in its filing that it received a federal grand jury subpoena on May 9, the same day it announced the resignation of Laplanche, the company's founder and a well-known figure in the online lending business.

The events have raised broad questions about the transparency and viability of the burgeoning sector.

Lending Club said in the filing that it "intends to cooperate with the DOJ and the SEC." The company declined further comment.

The Department of Justice did not immediately respond to a request for comment.

Lending Club, a pioneer in marketplace lending, revealed last week that it had knowingly sold an investor more than $22 million of loans that the investor did not want. In addition, an application date on $3 million of those loans had been altered so that they would appear to meet the investors' requirement, the company said.

"Certain personnel apparently were aware that the sales did not meet the investor's criteria," the company wrote in its filing. The company subsequently repurchased the loans.

Monday's filing indicated that Lending Club had repurchased an additional $3.8 million in loans during the first quarter that did not meet investor criteria, but it provided no further details.

The filing also revealed that Laplanche and Chief Financial Officer Carrie Dolan had used some of their company shares to secure personal loans, which the board discovered in an internal review. In January 2016, the reduction in the company's share price forced Laplanche and Dolan to refinance.

Lending Club said the company was not involved in these personal loans and did not have to give approval.

On Monday, acting chief executive Scott Sanborn sent a letter to investors, aiming to assure them of greater transparency in the company.

In March, he said, after Lending Club detected changes in the application dates of 361 loans, it hired an outside firm to audit about 673,000 loans sold to investors over the last two years.

As many as 99.99 percent of the loans audited showed no sign of having being altered, Sanborn said, adding that the company was also stepping up oversight of loans, doing more to detect tampering and retrain employees.

Shares of Lending Club are down more than 44 percent since prior to Laplanche's resignation and 64 percent for the year, reaching a low of $3.24 on Monday. The company raised $1 billion in its December 2014 initial public offering, when it sold shares for $15 apiece.

"Lending Club had a squeaky-clean brand," said Peter Renton, founder of LendIt, which organizes events for the online lending industry. "People did trust them and they have lost that trust." (Reporting by Heather Somerville in San Francisco and Rishika Sadam in Bengaluru; Additional reporting by Michael Erman in New York; Editing by Jonathan Weber and Clarence Fernandez)