The FBI raided the headquarters of United Development Funding, a large sponsor of real-estate investment trusts and other vehicles, sending shares of UDF's largest fund, United Development Funding IV, tumbling 54% on Thursday before trading was halted.

According to Tom Class, special agent in charge of the bureau's Dallas Division, FBI agents are conducting "law-enforcement activity" at United Development Funding's office. The FBI wouldn't comment further on the activity.

The FBI issued subpoenas to executives of the company and took materials from the premises, according to someone close to the matter.

A call to UDF's offices wasn't returned.

UDF, which has raised about $1 billion, mostly from individual investors, was the subject of a Wall Street Journal article on Tuesday pointing to risks related to the company.

The Journal's article highlighted UDF's concentrated lending. While it also operates in North Carolina, South Carolina and Florida, 99% of the UDF IV fund's portfolio consists of loans made to borrowers in Texas, according to the fund's most-recent quarterly filing. Other vehicles are highly concentrated too.

On its website, UDF had said "we operate in select markets across the country." UDF IV, which began as a nontraded REIT in 2009 and listed shares in 2014, said in an investor presentation that it enjoyed "portfolio diversification by submarket and loan type."

Further, 67% of the balance of its loans was made to a single borrower, Texas-based Centurion American Development, or Centurion's affiliates, as of Sept. 30, according to the fund's filings. UDF makes clear in its filings and on its website how many of its loans are made to Centurion and its affiliates.

Write to Gregory Zuckerman at gregory.zuckerman@wsj.com and Peter Grant at peter.grant@wsj.com

(END) Dow Jones Newswires

February 18, 2016 15:55 ET (20:55 GMT)

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