The owner of the company, Alan Fortich, 44; his brother, Alvaro Fortich Jr., 32, who worked there as an inspector; and the four other defendants surrendered on Thursday at the district attorney’s office.

Richard R. Leff, a lawyer for American Standard and Alan Fortich, said Wednesday that the accused “vehemently deny the allegations contained in the indictment.”

The indictment, which was unsealed on Wednesday, grew out of an investigation by the district attorney’s labor racketeering unit into Testwell, long the largest such company in the New York area. That inquiry began in early 2008, and later that year, after prosecutors concluded that Testwell had falsified results, the Metropolitan Transportation Authority selected American Standard to replace Testwell on the agency’s $4.3 billion Second Avenue subway project. American Standard also worked on the $2.1 billion extension of the No. 7 line.

But within months, American Standard had also come under scrutiny by the district attorney’s office, which investigated the two companies along with the city’s Department of Investigation and the inspectors general of the transportation authority, the Port Authority of New York and New Jersey and the School Construction Authority. Prosecutors won convictions last year against Testwell, its owner and vice president, and seven other officers and employees. The owner, V. Reddy Kancharla, was sentenced to 7 to 21 years in prison.

Concrete testing of the kind American Standard and Testwell do is a basic safety measure at construction sites, and investigators found irregularities in Testwell’s work at Yankee Stadium, One World Trade Center and other sites around the city.

The investigation began after Thacher Associates, a construction monitor hired by the Yankees and the Port Authority, uncovered irregularities in the concrete testing at Yankee Stadium and at ground zero, law enforcement officials and others briefed on the inquiries have said. The monitors gave the information to the Manhattan district attorney’s office, setting off the investigation in 2008.

Concrete is tested in a range of construction projects, like office towers, apartment houses, bridges and subways. Testing companies are supposed to conduct a variety of exams to make sure that concrete was properly mixed and met standards for strength and durability.