How do I report illegal work (under the table payment) to the IRS?

I am aware of a person regularly hiring people to do construction work but there is no record keeping, tax withholding etc. One of the workers was injured and has no workman's comp.
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If you suspect or know of an individual or company that is not complying with the tax laws, you may report this activity by completing Form 3949-A. You may fill out Form 3949-A online, print it and mail it to:

Internal Revenue Service
Fresno, CA 93888

If you do not wish to use Form 3949-A, you may send a letter to the address above. Please include the following information, if available:

Name and address of the person you are reporting
The taxpayer identification number (social security number for an individual or employer identification number for a business)
A brief description of the alleged violation, including how you became aware of or obtained the information
The years involved
The estimated dollar amount of any unreported income
Your name, address and daytime telephone number
Although you are not required to identify yourself, it is helpful to do so. Your identity can be kept confidential.
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Construction work is tricky. First of all, a company or person may hire someone else to work on a construction job and not have to withhold taxes, etc. This is called contract labor or Independent Contractor. As such, the person getting hired is responsible for all applicable taxes. Also if you are hired as an IC you are responsible for workman's comp injuries unless the hiring company or person is responsible for the accident.
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I assume you want the reporting to be anonymous. I don't know if the IRS takes anonymous reports or, if they do, whether they have the manpower to follow up on each report. But they aren't the only ones you could tell. The state's not getting its income tax either. Or, how about OSHA? Maybe the local newspaper would like to do an exposé.
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Many construction workers do work as independent contractors. They're hired on a job-by-job basis for a specific task, such as framing a home or doing the drywall work. They are properly responsible for their own taxes, insurance, etc. There's nothing to "report."
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It is my understanding that if workman's comp laws don't apply he can hire a regular lawyer and there is no limit on the law suit.

If he can find a lawyer who thinks it is a good case.
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If you report them, you can actually get some of the money recovered if you help the IRS get money back. Their web site has a page for reporting tax cheats.
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dont report it.. let them deal with it.. it's there business and the two of there issues.. not yours.. I think the guy that was injured should figure it out himself
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Just call them!
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Call their 800 number, the one that ends in 1040, they shall fall like the locusts they are