A whodunnit:

Mr. Jones, a well known, wealthy businessman arrives home from work around 5pm one Saturday afternoon to find a ransom note at his door; "If you want to see your son again, tonight you will bring $250000 to the largest tree in the park and leave it in the bag there."

He immediately calls the police, who begin investigating.
When questioned about everyone who was near his house that day, he replies that his wife had been home during the day as it was her day off.
His neighbour's son had his birthday party as well, which Mr. Jones' son had attended. At the party there had been Bobo the clown to entertain, and the neighbour, but otherwise, no other adults.
He also said that a plumber had been around near 4pm to fix the kitchen sink, as the pipe beneath had sprung a leak.

The policeman began questioning these people and they gave the following accounts:

Mr. Jones' neighbour:
"Yeah, my kid had a party, there were 20 screaming children until about 4, when they started to leave. Most of the kids lived on the same street, so they just walked home.
There was a clown here too, "Bozo", or something like that. I thought he was rubbish; the kids liked him though. He kept making these dumb balloon animals, he'd puff them up making stupid faces. He left around 4:30, as he had to pack up his stuff. I left at around 4:20, to drop off the kids that don't live on our street."

Mr. Jones' wife:
"I didn't even notice, I thought he was still next door. The plumber arrived at 4, and fixed the kitchen sink and then my husband came in waving this ransom note and I fainted. I was out back in the garden for most of the day, which is on the other side of the house to where the party was, so I barely heard them all day."

Bobo the clown:
"The kids were good, they loved my balloon tricks. I don't remember the kid you're talking about though, there were about 50 of them. The guy's other kid is having a party next month, so maybe he'll ask me to come back. I wouldn't mind doing a party for the guy next door though, he lives in a mansion. Must have a ton of dough to waste on clowns."

Plumber:
"I arrived to do some work on their kitchen around 4. It was a pretty easy job, only took about half an hour, but when I was leaving, I heard a bit of a noise next door. The fence was too high for me to see over, but I saw a balloon rise into that big tree, so I climbed up to have a look. I saw a clown muscle a kid into the back of a van and drive off.

The policeman arrested one of these people immediately after taking their account. Who was it, and why?