Its investigation found that partners at Thornton Law Firm donated nearly $1.6 million to

A Massachusetts law firm has been accused of funneling funds to the Democrats through an massive and illegal straw-donor scheme.

A growing number of party politicians are now returning donations from Thornton Law after it was found that the firm's partners received bonuses that matched their political donations in what may be one of the largest schemes ever revealed.

An investigation found that partners donated nearly $1.6 million to mostly Democratic committees and candidates, including Democratic presidential nominee Hillary Clinton, from 2010 through 2014.

During that same period, the lawyers received $1.4 million listed as 'bonuses,' including more than 280 in amounts that precisely matched their donations.

Some of them were supposedly reimbursed just 10 days after their money was sent to the party.

The small firm, which has 10 partners, reportedly also made donations to several other top Democrats including President Barack Obama, Sen. Elizabeth Warren and Sen. Harry Reid, to name a few.

The donation reimbursement program was exposed by the Boston Globe's Spotlight team and the Center for Responsive Politics.

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Top Democrats including Sen. Elizabeth Warren (left) and Hillary Clinton (right) are among a number of Democratic candidates who received donations from Massachusetts law firm Thornton Law Firm, which is accused of massive straw-donor scheme

An investigation found that partners at the Thornton Law Firm (pictured) donated nearly $1.6 million to mostly Democratic committees and candidates from 2010 through 2014, and during that period lawyers received $1.4 million listed as 'bonuses,' many matching those donations

A straw donor scheme is when a donor avoids legal limits on political donations by funneling money into campaigns using other people's names, and then reimburses donor for their donations.

In this case, the lawyers allegedly used part of the money they were handed over as partners as donations.

They were then reimbursed the funds.

A spokesman for the Thornton Law Firm said its donation reimbursement program was reviewed by outside lawyers and complied with relevant laws.

However, campaign finance experts said it raises numerous red flags because reimbursing people for their political donations is generally illegal.

'If you give a donation and then somebody else reimburses you for that contribution, that is a clear violation of the spirit and the letter of the law at the state and federal levels,' Scott Allen, Boston Globe's Spotlight editor, told CBS News.

According to federal law, partnerships like Thornton Law Firm are limited to a maximum donation of $2,700 per candidate.

But the firm reportedly used its individual partners as straw donors, which allowed it to donate money to campaigns above the legal limit.

'Straw donor reimbursement systems are something both the FEC and the Department of Justice take very seriously, and people have gone to jail for this,' Center for Responsive Politics editorial director Viveca Novak told CBS News.

A Thornton spokesman claimed the bonuses are legal because the partners paid them with their own money, coming out of each of their stake in the firm.

A spokesman for the Thornton Law Firm said its donation reimbursement program was reviewed by outside lawyers and complied with relevant laws

'It was a voluntary program which only involved equity partners and their own personal after-tax money to make donations,' the spokesman told CBS News in a statement.

Thornton Law Firm's attorney, Brian Kelly, told the Globe that 'while we see no need to do so, if individual politicians want to return donations, that certainly is up to them.'

On Monday, Clinton was one of several Democrats to vow that she would return thousands of dollars in donations from the firm, which is reportedly one of the country's largest political donors.

Instead of giving the money back to the firm, she will return the donations she has received to the US Treasury, a Clinton campaign official told The Boston Globe.

While it is not clear how much money she will be returning, Center for Responsive Politics show she received at least $20,000 from lawyers at the firm and their spouses during her presidential campaign.

During its investigation, the Spotlight team and the Center for Responsive Politics looked at donations from three of the firm's partners, including Michael Thornton (pictured)

Following the same move, Jason Kander's Missouri Senate campaign said Monday that it sent a $25,000 check to the U.S. Treasury after learning the firm might have violated federal election law with its donation reimbursement program.

Kander, who is Missouri's secretary of state, became aware of the situation on Sunday and immediately gave the money to the Treasury, his campaign spokesman Chris Hayden said.

'We thought it was appropriate to give the money to taxpayers rather than return it to the firm,' Hayden said in an email.

Democratic New Hampshire Gov. Maggie Hassan has said she is returning $51,000 in donations, including $13,000 to Hassan's gubernatorial campaign two years ago and $38,000 to support her current bid to unseat Republican U.S. Sen. Kelly Ayotte.

'We had no idea about the practices inside this firm, and we assume that as the Globe reported, none of the other Republican or Democratic candidates who received contributions knew either,' Hassan campaign spokesman Aaron Jacobs said.

'We will be returning the contributions from this firm.'

On Sunday, Democratic U.S. Senate candidate Russ Feingold's campaign said he returned a $45,000 donation.

Feingold is running against Republican Sen. Ron Johnson, whose campaign said Feingold was happy to take the contributions until he was caught.

It was reported on Monday that Democratic U.S. Rep. Peter Welch was returning $37,900 in donations from the firm.

Welch is seeking re-election as a Democrat and as a Republican, and faces a third-party challenger.

Above are Democrats who received donations from the firm. Several, including Sen Jason Kander, Sen. Maggie Hassan and Sen. Russ Feingold, have since returned the donations

A review of the records Welch found that since 2008 he had received the donations from Michael Thornton, his wife and employees of the firm, his office said.

'Congressman Welch was disturbed to read the Boston Globe story,' Welch chief of staff Bob Rogan said.

'Consistent with the paper's reporting, he was unaware of the law firm's internal practices related to campaign contributions. In accordance with (Federal Election Commission) guidelines for situations like this, we will immediately return these contributions.'

Boston Mayor Martin J. Walsh said his campaign staff is reviewing thousands of dollars in donations his campaigns have received from the firm.

He told the Globe he will immediately return more than $15,000 received this year.

Democratic U.S. Sen. Jon Tester has also turned over nearly 52,000 to the US Treasury in questionable campaign donations from partners at the firm.

Thornton donations were made to Tester's campaign from 2010-12 and he had not accepted any money from the law firm since then, Tester spokeswoman said.

On Monday, Republican U.S. Senate candidate Scott Milne called on Democratic U.S. Sen. Patrick Leahy to return $5,000 in contributions from the law firm from 2003.

He said Leahy should follow the lead of Welch and other Democrats 'who have recognized the impropriety' of keeping the contributions.

The donation reimbursement program was exposed by investigative reporters on the Boston Globe's Spotlight team (featured in the movie Spotlight, pictured) and the Center for Responsive Politics

'It is shameful, but typical, that Sen. Leahy refuses to return this tainted money,' Milne said in an emailed statement.

Leahy's campaign said it believed Milne was referring to three donors that collectively gave about $5,000 in August 2003, and it accused Milne of 'keeping his 100 percent negative campaign streak alive.'

'Rather than focusing on the issues facing Vermonters, Mr. Milne is desperately grasping at any attack he can in hopes that it will somehow damage Senator Leahy's reputation,' Leahy campaign spokesman Jay Tilton said.

'Senator Leahy has not received any contributions from this law firm in more than a decade.'

Meanwhile Warren, who has received $129,520 from the firm since 2007, told the Globe she will not give the money back until authorities find they were illegal.

During its investigation, the Spotlight team and the Center for Responsive Politics looked at donations from three of the firm's partners, including David Strouss, Garrett Bradley and Michael Thornton.

Allen said he is not 'confident' the program ran by Thornton is isolated to just their firm, noting a number of parties have come forward saying their companies also have a similar program.

'So the issue is always, can you prove it?' Allen told CBS News.