One crime is obstruction of correspondence, 18 USC 1702,
Whoever takes any letter, postal card, or package out of any post
office or any authorized depository for mail matter, or from any
letter or mail carrier, or which has been in any post office or
authorized depository, or in the custody of any letter or mail
carrier, before it has been delivered to the person to whom it was
directed, with design to obstruct the correspondence, or to pry into
the business or secrets of another, or opens, secretes, embezzles, or
destroys the same, shall be fined under this title or imprisoned not
more than five years, or both.
This matter would be investigated by the US Postal Inspection service, and the federal government may elect to prosecute the individual. This assumes that the passports were in an approved mailbox: a slot through the door is not an authorized depository.
A passport is property of the US government, see 22 CFR 51.7. There is a law, 18 USC 641, against doing that, specifically
Whoever embezzles, steals, purloins, or knowingly converts to his use
or the use of another, or without authority, sells, conveys or
disposes of any record, voucher, money, or thing of value of the
United States or of any department or agency thereof, or any property
made or being made under contract for the United States or any
department or agency thereof; or Whoever receives, conceals, or
retains the same with intent to convert it to his use or gain, knowing
it to have been embezzled, stolen, purloined or converted—
Shall be fined under this title or imprisoned not more than ten years,
or both; but if the value of such property in the aggregate, combining
amounts from all the counts for which the defendant is convicted in a
single case, does not exceed the sum of $1,000, he shall be fined
under this title or imprisoned not more than one year, or both.
Dept. of State has a web page telling you how to report a stolen passport: note that reporting a passport as stolen means it will be cancelled.
There are criminal statutes that prevent misuse of passports, but only one that comes close to covering the aforementioned confiscating of a passport, namely 18 USC 1545, which says
Whoever violates any safe conduct or passport duly obtained and issued
under authority of the United States shall be fined under this title,
imprisoned not more than 10 years, or both.
This is residue from the 1790 Crimes Act (discussed here), the point mainly being to protect ambassadorial rights – in those days, passports were not a currently-obligatory travel document. There appears to be no record in this law being applied against passport-theft.