An attempt by lawyers for victims of the Interstate 35W bridge collapse to pull back the blanket of secrecy over the investigation into its cause was blocked Thursday by federal law and a Hennepin County judge.

The law firm Schwebel, Goetz & Sieben, which represents several collapse victims and their families, sought information about the collapse from Wiss, Janney, Elstner Associates Inc., a Chicago-area engineering firm hired by the state to look into the reasons the bridge suddenly gave way during rush hour Aug. 1, killing 13 and injuring about 100.

But citing National Transportation Safety Board restrictions, Hennepin County District Judge Herbert Lefler ruled that the information does not have to be released without the approval of federal investigators.

"(Wiss Janney), which has contracted with the State of Minnesota, is an agent of the state and considered by the NTSB to be a participant in the investigation of the bridge collapse," Lefler wrote. "As such, all data collected by (Wiss Janney) for the investigation is non-public. ... Plaintiff has no right to the data it requests at this time."

So far, the NTSB has only offered clues into the course of its investigation, outlining several broad areas it is studying. They include the design of large steel plates that held pieces of the bridge together, the weight of construction materials and equipment on the bridge at the time of collapse and the effects of an automated anti-icing system installed on the bridge in 1999.

The NTSB has declared the Minnesota Department of Transportation a party to its investigation, which might not be completed for another year. Federal rules prohibit the release of information unless approved by the NTSB's investigator-in-charge.

The state argued that Wiss Janney is also subject to those rules, but Schwebel, Goetz & Sieben sought a ruling that information developed by the firm should be released under Minnesota's Data Practices Act.

"We're going to appeal this. It's outrageous that MnDOT should be allowed to hide under the shield of secrecy of the NTSB," lawyer Jim Schwebel said. "What are they afraid of disclosing?"

Victims' lawyers have been frustrated by a lack of access to pieces of the collapsed bridge and are concerned that any independent investigation will be compromised without prompt access to the materials.