The federal government is now investigating one of the most famous tours in the world because of an NBC4 investigation. The I-Team series documented potential safety hazards of Hollywood tour buses, including showing how some seatbelts have been modified or removed altogether. The series also looked at how the buses have been customized so that they are "topless," leading experts and officials to question their ability to protect passengers in the event of an accident.

The United States Department of Transportation has mailed certified letters to more than two dozen tour bus companies stating it believes the convertible tour buses lining Hollywood Boulevard are not safe and should not be taking passengers anywhere.

"It instantly became clear to us that this was a safety issue that we needed to take action," said Bryan Thomas, the communications director for the National Highway Traffic Safety Administration.

"We're advising the tour operators that we don't believe these vehicles are safe in their present condition and they shouldn't be transporting passengers," Thomas added.

NHTSA also issued a "special order" to each tour bus operator, demanding information about the customized tour vans, so that it can investigate potential violations. It is specifically asking about whether seatbelts were modified, and details about the roof removal. Thomas told NBC4, "We're very curious as to how the modifications were made, who made those modifications and we'll be collecting a lot more information about that in the coming weeks."

"Our main concern is making sure no crashes, no accidents happen in these vehicles; no one is injured," Thomas continued. "We're taking action to prevent any tragedy like that."

California Assemblyman Adrin Nazarian has told the I-Team he's taking action to better regulate the Hollywood tour industry as well. He plans to introduce at least one new piece of legislation on the first day of the Assembly's next session in January.

Dozens of the "open" tour vans transport thousands of tourists and residents every week into the hills above Hollywood and Beverly Hills to point out landmarks and celebrity homes.

The NBC4 I-Team interviewed one mechanic who cuts the tops off vans for several Hollywood tour companies. Pedro Rodriquez of "Rudy's Custom Tops" said he does reinforce the structure, but added that he cuts out the shoulder belts that are attached to the roof.

Many of the modified vehicles used for "open" Hollywood tours are Ford vans. When contacted by the I-Team, a Ford spokeswoman said it's the first the company has heard of its vehicles being customized in this way, and added that the mechanic or auto shop that did the modifications would be responsible for adhering to all safety standards.

Ford added that "Rudy's Custom Tops" is not one of the company's "approved [vehicle] upfitters," or modifiers.

The National Transportation Safety Administration has also sent a special order to "Rudy's Custom Tops," urging the owner "to discontinue making such modifications," and warning owner Rodriguez that by altering the vehicles, he could be violating federal safety standards.

Companies that don't respond to the National Highway and Traffic Safety Administration's special order by Sept. 19 could be subject to civil penalties of up to $21,000 per day.

The National Transportation Safety Administration is now urging tourists and residents to steer clear of the modified tour vans.

As the I-Team first revealed, many of the celebrity homes they point out are lies anyway.