On Wednesday morning, Johnston told CNN that the stamp suggests that the version of the documents he received didn't come from the Internal Revenue Service. He said he doesn't know for sure who sent him the documents.
The documents came from someone who had access to them, either through an accounting firm who prepared the documents or in litigation or a regulatory proceeding, Johnston told CNN. He said the firm had no comment on the documents.
It has been reported that Trump posed as his own publicist under the names John Miller and John Barron. The real estate mogul has denied this, despite recordings that reveal the two had remarkably similar voices.
Johnston said Tuesday evening that Trump has a "long history of leaking material about himself when he thinks it's in his interests." While that is a possibility, he told CNN Wednesday morning that "the anger with which the White House responded suggests" it isn't likely Trump leaked his own tax document.
The tax documents aired on MSNBC match figures given by the White House in a statement Tuesday, which said Trump had paid $38 million in taxes on income of more than $150 million for 2005.
The documents indicate that Trump paid an effective tax rate of about 24 or 25 percent. But Trump could have paid a lot less, if not for the alternative minimum tax, something he has said he'd eliminate.
Trump paid "no more tax than legally required," a spokesperson for the White House said in response to MSNBC's report. The full statement is below:
"Before being elected President, Mr. Trump was one of the most successful businessmen in the world with a responsibility to his company, his family and his employees to pay no more tax than legally required. That being said, Mr. Trump paid $38 million dollars even after taking into account large scale depreciation for construction, on an income of more than $150 million dollars, as well as paying tens of millions of dollars in other taxes such as sales and excise taxes and employment taxes and this illegally published return proves just that. Despite this substantial income figure and tax paid, it is totally illegal to steal and publish tax returns. The dishonest media can continue to make this part of their agenda, while the President will focus on his, which includes tax reform that will benefit all Americans."
— MSNBC contributed reporting.
Disclosure: NBCUniversal is the parent company of MSNBC and CNBC.