P.R. Laws Ap. tit. 32A, § III, Rule 58.3
(a) Caption. — The complaint shall contain a caption as provided in Rule 8.1, except that the plaintiff shall name as defendants the property, designated generally in kind, quantity and location, and at least one of the owners of some part of or interest in the property.
(b) Contents. — The complaint shall contain a short and plain statement of the authority for the taking, the use for which the property is to be taken, a description of the property sufficient for its identification, the interests to be acquired, and as to each separate piece of property a designation of the defendants who have been joined as owners thereof or of some interests thereon. Upon the commencement of the action, the plaintiff need join as defendants only the persons having or claiming an interest in the property whose names are then known, but prior to any hearing involving the compensation to be paid for a piece of property, the plaintiff shall join as defendants all persons having or claiming an interest in that property whose names can be ascertained by a reasonably diligent search in the Registry of Property, considering the character and value of the property involved and the interests to be acquired, and also those whose names have otherwise been learned. All others may be made defendants under the designation “Unknown Owners”. In case the property lacks possession or ownership title the person or persons appearing as owners of the property in the tax receipt or in any other similar document establishing a title shall be added as defendants. Process shall be served as provided in Rule 58.4 upon all defendants, whether named as defendants at the time of the commencement of the action or subsequently joined, and a defendant may answer as provided in Rule 58.5. The court meanwhile may order such distribution of a deposit as the facts warrant.
(c) Filing. — In addition to filing the complaint with the court, the plaintiff shall furnish to the clerk at least one copy thereof for the use of defendants and additional copies at the request of the clerk or of a defendant.