Large object moved slowly over the city

This not a first hand report.  I've had this event in my memory for many years, and I am wondering if any one can substantiate or add detail to what I'm calling an event.
I believe it happened between about '68 and the early 70's.  My father worked as the attendant at one of the few all night gas stations along the old Rt 66 as it passed through Joplin, MO -- locally known as East 7th Street.  The station sat near the crest of a very long up-slope and so at night one could see back across the city quite well.  He was the introverted, serious type person.  Many of the local cops patrolling the city made this station a regular night-shift break stop and they shared all the "cop gossip" with him.  He stated to me the following when I visited on a weekend.
One night he saw an illuminated metallic craft moving slowly just above rooftop level across the city.  The motion was steady and it exhibited a controlled motion.  It apparently did not come directly over the gas station.  This activity continued long enough for several police cars to engage in trailing the craft around the city, as he saw the roof lights on their cars in the distance and heard sirens.  He said that the craft eventually gained altitude and simply faded into the night sky -- there was no sudden vanishing, it just flew off very calmly.
Later that same night, many of the cops converged at the gas station and began relating to each other what they had witnessed.  My father, of course, heard much of the chatter between officers and on the radios.  All agreed that they had seen a very large metal craft that simply glowed with no apparent source of light.  Some reported what appeared to be windows and various details about the skin, much as you might attempt to describe the surface of any vehicle after having been up close.
After a few nights, the various cops on duty the night of the sighting resumed their regular break stops at the gas station.  All told my father a similar thing: They had all been instructed to forget that they had seen any thing and to speak no more of it.
Dad said that the Joplin Globe Newspaper ran an article on the UFO as soon as they gathered the early police reports, but then the story was never given any additional press by the newspaper.
I repeat that this was told to me by my father years ago.  I buried this for so long that I lost track of when it happened.  I wish that I'd had enough presence of mind to have at least written down the date.
Witness indicates that the date of the event is approximate.  PD