The FBI on Thursday raided the office of a Grapevine company that has financed more than $1 billion in residential development across Texas, but some say the company operates as a Ponzi scheme.

Agents were seen carrying boxes out of United Development Funding on the 1300 block of Municipal Way and loading the boxes into large trucks.

UDF has acknowledged it has been under investigation by the U.S. Securities and Exchange Commission and said it was cooperating, but added no specific charges of wrongdoing have been made.

The FBI raid was the first indication that a criminal investigation into the company was under way.

A hedge fund founded by Dallas investor Kyle Bass, Hayman Capital Management, claims in a website that, "UDF exhibits characteristics consistent with a Ponzi scheme."

On Feb. 5, UDF’s chief executive officer, Hollis Greenlaw, posted an online message to shareholders accusing Hayman of making “false and misleading statements about our company.”

A number of class action lawsuits have been filed against UDF on behalf of investors.

Candy Evans, who writes a blog on Dallas real estate called candysdirt.com, said UDF appears to be in financial danger.

"It would look like they're in trouble," she said. "And it would look like whoever invested in it is probably not going to get anything out of it."

The company's already-falling stock price nosedived so fast on Thursday that NASDAQ suspended its trading.

Kathy Colvin, a spokeswoman for the U.S. Attorney’s office in Dallas, declined to comment on the FBI raid.

FBI spokeswoman Allison Mahan confirmed that agents were at the firm's offices conducting a law enforcement operation, which she declined to characterize as a search warrant.

Nobody from UDF could immediately be reached for comment.

NBC 5's Jocelyn Lockwood contributed to this report.