Wash. Rev. Code § 59.18 § 59.18.510

(1)(a) Any person whose life, safety, health, or use of property is being injured or endangered by a tenant's gang-related activity, who has legal standing and resides, works in, or owns property in the same multifamily building, apartment complex, or within a one-block radius may serve the landlord with a ten-day notice and demand that the landlord commence an unlawful detainer action against the tenant. The notice and demand must set forth, in reasonable detail, facts and circumstances that lead the person to believe gang-related activity is occurring. The notice and demand shall be served by delivering a copy personally to the landlord or the landlord's agent. If the person is unable to personally serve the landlord after exercising due diligence, the person may deposit the notice and demand in the mail, postage prepaid, to the landlord's or the landlord's agent's last known address.
(b) A copy of the notice and demand must also be served upon the tenant engaging in the gang-related activity by delivering a copy personally to the tenant. However, if the person is prevented from personally serving the tenant due to threats or violence, or if personal service is not reasonable under the circumstances, the person may deposit the notice and demand in the mail, postage prepaid, to the tenant's address, or leave a copy of the notice and demand in a conspicuous location at the tenant's residence.
(2)(a) Within ten days from the time the notice and demand is served, the landlord has a duty to take reasonable steps to investigate the tenant's alleged noncompliance with RCW 59.18.130 (9). The landlord must notify the person who brought the notice and demand that an investigation is occurring. The landlord has ten days from the time he or she notifies the person in which to conduct a reasonable investigation.
(b) If, after reasonable investigation, the landlord finds that the tenant is not in compliance with RCW 59.18.130 (9), the landlord may proceed directly to an unlawful detainer action or take reasonable steps to ensure the tenant discontinues the prohibited activity and complies with RCW 59.18.130 (9). The landlord shall notify the person who served the notice and demand of whatever action the landlord takes.
(c) If, after reasonable investigation, the landlord finds that the tenant is in compliance with RCW 59.18.130 (9), the landlord shall notify the person who served the notice and demand of the landlord's findings.
(3) The person who served the notice and demand may petition the appropriate court to have the tenancy terminated and the tenant removed from the premises if: (a) Within ten days of service of the notice and demand, the tenant fails to discontinue the gang-related activity and the landlord fails to conduct a reasonable investigation; or (b) the landlord notifies the person that the landlord conducted a reasonable investigation and found that the tenant was not engaged in gang-related activity as prohibited under RCW 59.18.130 (9); or (c) the landlord took reasonable steps to have the tenant comply with RCW 59.18.130 (9), but the tenant has failed to comply within a reasonable time.
(4) If the court finds that the tenant was not in compliance with RCW 59.18.130 (9), the court shall enter an order terminating the tenancy and requiring the tenant to vacate the premises. The court shall not issue the order terminating the tenancy unless it has found that the allegations of gang-related activity are corroborated by a source other than the person who has petitioned the court.
(5) The prevailing party shall recover reasonable attorneys' fees and costs. The court may impose sanctions, in addition to attorneys' fees, on a person who has brought an action under this chapter against the same tenant on more than one occasion, if the court finds the petition was brought with the intent to harass. However, the court must order the landlord to pay costs and reasonable attorneys' fees to the person petitioning for termination of the tenancy if the court finds that the landlord failed to comply with the duty to investigate, regardless of which party prevails.