Sycamore Partners, a private-equity firm that focuses on retail investments, said it’s interested in acquiring the clothing chain Express Inc. after amassing a 9.9 percent stake in the company. The retailer's shares soared on the news.

In a letter to Express’s board, Sycamore said it would like to perform due diligence to determine a valuation for the retailer. The investment firm said it would submit a bid within 30 days of getting access to Express’s books.

“We believe that this process will result in an acquisition proposal that benefits all of the company’s constituents, including its stockholders, customers, vendors and associates,” Stefan Kaluzny, Sycamore’s managing director, said in the letter. “Given the strategic and operational challenges faced by specialty retailers generally and the company in particular, a fully financed, binding, all-cash offer to acquire the company would be a valuable alternative for the company’s board of directors and stockholders to consider.”

Marisa Jacobs, a spokeswoman for Columbus, Ohio-based Express, declined to comment on the offer, saying her company hadn’t yet received all of the documents and needed to review them. Express has a market valuation of $1.14 billion.

Shortly after 6 p.m. in New York Thursday, Express shares were up 20 percent at $16.30, after falling 2.6 percent to end the regular session at $13.55.