New York (CNN) The substance sent in a white envelope addressed to Donald Trump Jr. on Monday was determined to be cornstarch, a law enforcement source tells CNN.

Another law enforcement source said the letter, postmarked in Boston, was addressed to Trump Jr. but was actually sent to the Manhattan apartment of his mother-in-law, Bonnie Haydon. Vanessa, Trump Jr.'s wife, opened and handled the letter. The NYPD, citing preliminary information, said Monday that the letter had been sent to the Trumps' apartment.

Vanessa Trump, Haydon and a Secret Service agent were transported to the hospital for further evaluation, the second law enforcement source said.

Vanessa Trump complained of nausea. Ultimately, all three were released from the hospital unharmed.

The letter addressed to Trump Jr. ridiculed his intelligence and said that people hate him, the second source said.